anything below a dual 256 will suck.
seriously.
luckily for me, i have a scarily powerful computer, so i'm in no danger of crashes or lag.
i still lag slightly with full water effects, shadowing and HD lighting, but apart from that i'm fine.
i can't stand running a game on any setting below maximum.
it might work on a single 256, but if you're using a 128 or a 64, you're really just fucked. lol