I believe the S series uses a 2.5” hard drive so you will be able to upgrade that up to about 100gig yourself. There are threads on this site which explain how to open up the laptops.
The T uses the same 1.8” hard drive found in the TR series, and currently they’re really only available upto 40gig, although 60gig versions are available they’re not at all wide spread yet probably due to Apple using them all for iPods!
Last time I looked on ebay there weren’t that many of either T or S’s so you’ll be lucky to get to choose between XP home/pro. You might just have to ask each seller individually which it comes with if they haven’t listed it.