More RAM the better, especially for newer games, like Doom3 or UT2K4.
Although, I wouldn't pay Mesh's (I take it this is the same machine your mentioned in other posts) inflated price of £100, to purchase an extra 512mb.
Go here, click here £60 inc VAT. And it's really easy to fit.
Also on the subject, if you only get the 512mb in your ordered machine, make sure they supply one 512mb module and not 2x256mb modules. (They will accomodate this, if you ask when ordering)