I'd consider going with the HD4770 which are available right now on Newegg from several different makers, they use 80w of power at load and would be fine for the Antec 500 in crossfire. Also the VIA VT1708S doesn't have windows7 64bit drivers just yet, they have a post up at their site that they're "coming soon". Windows7 does have dated drivers that will work with that chipset so audio is functional (at least for me), but they're not current. There should be no decrease in fps with 8x-8x PCIex16 2.0 and the 4850 or 4770. You would see a slight decrease if you went 4870 or 4890 from what i've read.
I put these in an Intel Maximus II Formula, I built for a friend. Running 2kits totaling 8gb running at 1.85v stock voltage, using Vista Ultimate 64-bit OS. Also if you do wish to OC the ram, you can by changing the timings to 5-5-5-12 and increase the voltage to 2.1v to get them to run at 1066.