Xfx ati radeon hd 4890 causing reboots...won't post.

mobo - evga nForce 730i
CPU - Core 2 Duo Quad Core 6700
RAM - 8GB DDR2
PSU - 700W Modular Rocketfish
GPU - xfx ati radeon 4890 hd

When i pop the 4890 onto my mobo and start my pc, it doesn't even post. it beeps once and then reboots over and over. If i put another video card in (nvidia geforce 9800gtx ) it boots just fine. If i put the 4890 onto another motherboard, it works just fine. so...the video card works, the motherboard pci-ex16(gen2) slot works, but i can't get the two to work together. I have tried both the 700w rocketfish PSU and a 650w corsair as well. both should be more than enough to handle the power needs. I called evga support and we went over a few cmos settings, but i have all of the cmos settings at their factory default and that seemed to square with evga support. I can't figure this one out! Any help would be appreciated..