With those motherboards your crossfire is going to cut you down to 4x in the second PCI-E x16 lane unlike SLI 32 that would give you true 16x times in each lane. If you are an ATI then I would get the single 2900XT 512 but you may want to get an NVIDIA GeForce 8800 GTS 640MB
Sapphire Radeon HD 2900XT 740MHZ 512MB 1650MHZ GDDR3 Dual DVI-I HDCP VIVO Video Card - $479.99
EVGA E-GEFORCE 8800GTS 500MHZ 640MB 1.6GHZ GDDR3 Dual DVI-I HDTV Out HDCP DIRECTX10 Video Card - $385.99
The 2900 XT does not compare to the 8800 GTS 640MB. By getting the 8800 GTS 640 MB you are saving almost $100.00
You can do a comparison here.
VGA Charts 2007 | Tom's Hardware