I don't think there many games on the market that will take advantage of more than 2gb of ram.
But I'm thinking on a future proof kinda level if 4gb costs like £65 you might aswell go with that option then you will have a machine that in terms of performance and future proofing will last you longer.
4gb of ram comes in handy if your the kinda user who likes to have Various applications running at the same time i.e CS3 and Dreamweaver
Bearing in mind you will need the 64 bit version of Vista to support 4 gigs properly.
Processor wise I personally would go with a Athlon X2 5600 there cheap , perform well and the bonus is it's a socket AM2 you then have the option to upgrade to a Phenom,