newegg.com and zipzoomfly.com and buy.com are good places to get a new HD. Look for a Hitachi or a Seagate, there've been some bad reports about Toshibas lately. Be sure to get a 5400 rpm model, because you'll see an immediate speed increase in HD access. When I upgraded to a 5400 rpm HD in my Ti-400, the TI didn't feel so old after all.
The other great improvement in an early model Ti, is a PMCIA wireless card (if you can find one cheap enough. When I put a first a Cisco Aironet 350 and later a Buffalo wireless card ($25) in the Ti-400 the difference was night and day from the original internal wireless card.