Paul Russell writes about his 3-yearly laptop replacement at IBM . It probably makes some sense to replace laptops periodically for a large company, but if you are buying for personal use then it makes sense to try and get a longer life out of an expensive machine. I think that aiming for 6 years is quite reasonable with today’s hardware – you should be able to buy a new machine now and have it last 6 years or buy a 3yo second-hand machine and hope to have it last 3 years (most second-hand laptops on sale in every place other than Ebay were trophies for managers and never had any serious use).
If you are going to buy a second-hand laptop the first thing to consider is PAE support. If you get a laptop without PAE support (I think that means all Pentium-M CPUs) then you will not have proper Xen support (it seems that all distributions have abandoned Xen support for PAE for the moment). This may not be a big deal if you don’t want Xen, but if you are a programmer then you probably do want Xen (even if you don’t realise it yet). The next issue is support for the AMD64 instruction set. 32bit laptops are going cheap at the moment but if you buy one you will be significantly limited as to what software you can run at some future time (my 32bit laptop is doing well at the moment apart from the lack of PAE support).
If you are buying a new laptop then the first thing to consider when planning a long life is the warranty. In my experience most computer gear does not need a long warranty, if it survives 3 months then it’ll probably last until it’s well obsolete. Laptops however periodically wear out if used seriously, I average one warranty replacement of a Thinkpad keyboard every two years and I have had a few motherboard replacements (the lighter Thinkpads flex and they eventually break inside if you use them on enough trains, trams, buses, planes, etc). On one of the Lenovo T series Thinkpads that I saw advertised (one that I would consider if I wanted a new laptop now) there was an offer to spend an extra $350AU to get extend the warrantee from 1 year to 5 years (according to my understanding of the confusing text on the web site) on a laptop that cost $3050. An increase in the purchase price of 12% for the extra warranty is a bargain (I know that for my use they would lose money on the deal). Repair of a laptop is generally very expensive, any serious damage to a laptop that is more than 18 months old will generally mean that the replacement cost is less than the repair cost.
The next thing to consider is the screen resolution. After purchasing a laptop you can upgrade the RAM and the hard drive, the CPU power of all modern machines is great enough that for most typical use it’s difficult to imagine any need to upgrade. But screen resolution is something that can never be good enough and can never be improved after purchase. Lenovo is offering T series Thinkpads with 1920×1200 resolution for $4000AU and 1680×1050 resolution for $3050AU. That’s 31% more pixels for 31% more money and seems like a good deal to me. I believe that a larger display can significantly increase productivity  so it seems that the extra expense would be a good investment if you plan to earn money from work you do on your laptop. As a point of reference a desktop monitor from Dell (who seems to be the cheapest supplier for such gear) with resolution of 1920×1200 will cost at least $1000AU.
The hard drive capacity should not be an issue, it seems that 100G is about the minimum size. The 60G drive in my current Thinkpad is adequate for my development work (including several Xen instances and some ISO files for a couple of distributions) so unless you plan to collect MPEG4 files of TV series and store them on your laptop I can’t imagine 100G being much of a limit. Also external storage is getting quite cheap, 2GB USB flash devices are now in the bargain bin of my local electronics store and USB attached hard drives with capacities of 40G or more are getting cheap. Also with a Thinkpad replacing a hard drive is really easy and does not risk damage to the drive or the rest of the laptop (I don’t know how well other brands rate in this regard).
For RAM you can buy a model with a large memory module in socket 0 (or attached to the motherboard). Adding new RAM later is easy to do. Just try and avoid purchasing a memory capacity that involves having all sockets filled with modules that are not of the maximum size – it’s annoying if you have to try and sell modules on Ebay after you buy a memory upgrade.
Finally one mistake I made in the past was to not get all the options for the motherboard. Make sure that every option for Ethernet ports and 802.11 type protocols is selected. It might sound like a good idea to save $100 or so on not getting one of those options, but if you end up repeatedly plugging a CardBus or USB device for many years you will regret it. Also external devices tend to break or get lost.
Rusty documents his laptop replacement as a time for spring-cleaning. I use LVM for the root filesytem on my Thinkpad so that I can easily install a new distribution (or a new version of a distribution) at any time. I’ve been through that spring-cleaning a couple of times on my current Thinkpad without needing new hardware.
From a quick view of the Lenovo site it seems that an ideal new Thinkpad that would last me 6 years would cost about $4500 while one that would last me 2 years would cost $1600 (and have a significantly lower screen resolution). A Thinkpad that would last 6 years and not be so great (but still better than the cheap option) would cost about $3500.
Update: One significant issue is the life expectancy of laptop batteries. If you use a laptop for mobile use (as opposed to just moving between desks occasionally) then you are probably familiar with the problem of laptop batteries that discharge after 10 minutes. Last time I checked the warranty on Thinkpad batteries was 1 year or 300 charges (whichever comes first). My experience is that after 300 full cycles a Thinkpad battery will only last for a small fraction of the original charge time. When buying a laptop I suggest getting a spare battery at the time of purchase. The spare battery may last longer than the battery that is shipped with the laptop and two batteries means that you have twice the number of charge cycles before they are both useless. Batteries apparently don’t last long if completely discharged, so charge them up before storing them and periodically charge them if they have been left unused for any length of time (maybe every second or third month). With a Thinkpad it seems quite safe to change the battery while the machine is plugged in to mains power and running (I expect that Lenovo doesn’t recommend this though). You should probably plan to have a battery die every three years of use (or sooner if you do a lot of travelling). So one spare battery may last you 6 years of use but you will need two spare batteries if you travel a lot.