Skunks unfortunatley buying a used laptop is throwing good money away in my opinion. What was top notch 3 years ago is now aging junk that will be unable to cope with the new programs and software that will be forced down our throats in a few short years time.
My advice is to save your loot and buy the best top of the range machine of today you can possible afford. By this I mean your looking at a 1 gig processer with at least 256k ram and a minimum of 10-15 meg HDD with a DVD.
Buying one now would maybe set you back quite a lot of money but at least in 5 years time it will still be of very good use and cope well in 2006.
Just look at computers like cars. Buying an old hack (always in the garage, spare part problems, etc) every 2 years is more expensive than buying a late-model that can last you 5 to 10 years.