5750 Won't Post

This has me baffled. Got an XFX 5750 for my kid's computer. It's an Asus A8R32-MVP Deluxe MB with an Opteron 185, and an Antec 650w Truepower Trio power supply. It currently uses an HD 3870. So I installed the 5750 when I got it last week (and I did hook up the 6-pin PCIE power connector) and turned on the pc. The card's fan spins up, but there's no signal to the LCD. System is obviously hung, so it's not as if it's booting up but just not getting the signal out. Tried it in the second PCIE slot, same thing. Tried the monitor in both DVI connectors, no dice.

So, I figured it was bad and sent it back. Got the replacement today and installed it...same deal. So, I pulled out my second HD 4850 out of my computer and installed it in their computer. Works fine. Put the 5750 in my computer, and it posts normally. My computer is a Gigabyte GA-X38-Ds4 with an Intel C2D E8400.

So, I guess the 5750 just isn't compatible with that motherboard? Seems strange to me...it isn't that old of a board.

By the way, the reason I upgraded the card is because of frequent BSOD in Sims 3 that they want to play. The crashes are all associated with the ATI graphics. So, I kept the 4850 in and it just crashed too. Maybe I need to jump to Nvidia?

BTW, the OS on both systems is Win 7 x64 Ultimate.