But u don't want to be playing at lower detail because you have less RAM do u? Surely its better to get 2x1GB. And as most games need recommend 1GB now in a few months the recommended will be 2GB so surely the 2x1GB is the better option as u can't really upgrade if you use 2x512 as more modules won't add enough. I suppose it depends on budget and persenal preference
I would say 2x 1Gig.... Thats what I have in my Shuttle box... All of my games run really well, and only lag when online servers lag.
ONLY problem I have had with my RAM is that when I rip music from CD's in iTunes, it seems that when I am ripping a scratched CD, it will throw a memory error, give me a BSOD (in Win XP Pro) then reboot...