Which upgrade would make the most difference?

Unless you have a 64-bit OS going 4GB of ram is a waste of cash since Windows will not see it (it's the limitations of a 32-bit OS). So i'd say stay with your current 2GB. As for the card, do you find that 640GB 8800GTS you currently have as insuffient? Those are great cards and dumping another $320 or so on another will just slighlty increase your FPS but such an investment is not worth the cash (i'm still one of those guys not truely sold on SLI). If you can wait, Nvidia is currently planning on bringing out the 9series cards between mid November to early December. Just have a feeling 3 months down the line your gonna be kicking yourself for going through all the trouble of selling your GTS, buying a GTX then drooling over the new 9series cards which will supposedly sport over one TeraFLOPS of shader processing power, DX10.1 (rumors saying current 8800 cards will not support this new DX10, waiting for Nvidia to confirm that), and DDR4 memory. This card WILL be released this year. Now they are rumors that a 8900 series card will hit the market but this is unconfirmed. With all these new Nvidia cards coming out that will most likely be around the same price range of that 8800GTX your looking to get i'd wait a bit bud and see what's what. Just one builder's opinion.

Thanks guys, sound advice. I didn't know 4Gb of RAM was for 64bit O/S only.

I'm not dissatisfied with my GTS. The only reason I was thinking about upgrading at all was in preparation for games like Bioshock and eventually Crysis (which I realise ideally needs Vista anyway which is a bit of a shame). I'm just not sure how much of a pounding a GTS will take with new games like this.

I think I'll take your advice and stick with what I've got until the 9x cards come out and Vista is a bit better.

Cheers

Graham

Save your money for now, buy one of the quad core 45nm cpu's when they come out. Eventually you will want to make the leap to vista 64 and 4-8 gb too. I have a similar setup to you but have 4gb in 64 bit, going from 2 to 4 gb doesn't make much diff, Yet.

I will prob wait till 9800 prices settle down n spring before upgrading from my 8800gtx.

I have been itching to go to raid0 raptors from my single, but such a hassle.