yeah at this point it is impossible to upgrade that computer. the mainboard is presumably the first thing to go is it is outdated and none of the other components are suitable to run a post-2008 game either... so you have to buy an entire new PC or a console. if you have a good HDTV/widescreen monitor at your disposal and want to save money buy an HD-console (the purpose of this term is to exclude Wii). if you on the other hand have only an old standard definition tv / CRT monitor then buy a gaming PC or the components from a trustable internet site that isn't related to DELL / Alienware / any major brands. you should be able to customize beginning with presets or even from scratch, those kinds of sites exist but i only know the germany-based ones

prices of components shouldn't be extraordinarily high or low, relatively close to the retail ones, and the company running the site shouldn't be brand-new or you risk problems with support, when they **** up your order or something's not working.
one last thing: ATI has better prices right now (than nvidia) and you should also strongly consider when you're planning to switch to win7 (unless you already have). if it isn't anytime soon then you don't need DX11 in your card and can go for the more refined end-of-cycle DX10 cards. like a 4890 or the gtx 285. they're are both gonna run crysis 2 on high settings according to crytek's claims of better performance on the same settings (both cards can run Crysis 1 nicely on high settings).