Here's my thoughts/opinions...
1) 512MB of RAM is probably more than you need, so I wouldn't worry about
the amount of RAM. It's the speed of the processors that will kill your
game playing experience.
2) In fact, AFAIK, if you put in the 2x256MB cards, because of the way the
SLI/Crossfire is done, it's equivalent to 1 fast 256MB card, not a fast
512MB card. See this link for some info on SLI:
http://en.wikipedia.org/wiki/Scalable_Link_Interface. Yes, I realize it's a
wiki, but you can go do your own research if you don't trust it.
3) Having lots of RAM but a slower card is a fool's game, because the card
can't keep up with the textures anyway, so it won't do you any good.
4) You're better off getting one fast card than two slower ones. If you
buy a single fast one, you can upgrade it later with another fast one. But
if you get two slower ones, you're pretty much maxed out.
5) Depending on the game, you may not be able to drive a game to dual 19"
monitors with either a single "large" card or two small ones. Oblivion, for
example, would probably require two fast cards to drive dual 19" displays
properly, and even then it may bog down at times. That's a combined
resolution of 2560 x 1024 (2x1280), which is a lot of pixels.
6) If you're not going to drive a game to both displays at the same time,
then you're just doing 1280x1024, and you'd probably do fine with a single
high-end card, like an ATI X1900xxx card (whatever you can afford). I run
Oblivion reasonably well on my X1800GTO (256MB of RAM), driving a 20"
monitor at 1280x800 (widescreen). It bogs down in some places, but not badly
after I've tweaked the graphics settings. Definately playable and
enjoyable.
If you want better input, here's some questions. Why do you need 512MB of
RAM? What are you using the system for? If it's for games, which ones? If
it's not for games, all of this is a moot point, because even a single low
end ATI 9250 can drive dual 19" displays in 2d mode.
Clint
