PC World have a Web special on Toshiba 320s for 115 GBP.
Most cost effective is a spotter's market (that is, computer fair - most of the attendees have spots). USB2 external case for 18 quid and then choose your drive at leisure - 80 GBP for a 250GB. 17 GBP extra buys a lot.
I should point out that I have no connexion with PC World and I find that they generally suck - but a name-brand drive should give some peace of mind.
As Conan says, 100 is Neolithic now and the extra space can/will always be used.
BOFH