Thanks for the comments - my monitor will be a 23" running at 1920 x 1080. In maybe two years the price for either of the cards should be around $100-$150, and I figure I will get another one so that my dual-GPU setup can still match the better single GPU cards available by then. So I definitely want to make sure I make the right choice with the first card (and will be of course using it to play new games until then).
I figure that the ATI 5850 should be able to Crossfire with even standard coolers, and work fine on a 700 W PSU w/ an efficient i5, as its voltage and heat seem to be quite low from the reviews here. I'm guessing, though, that SLI'ed dual FX470's will require more than a 700W PSU and probably a very good air cooled case or liquid cooling to run safely. If anyone here has a FX470, I would definitely like to know your experiences with its cooling and power requirements, mainly so I can rule it out