As far as I understand the answer is NO.
You can use any single ATI / nVidia PCI-e / other vendor video card in ANY single slot PCI-e ready, dual PCI-e slot SLi or Crossfire motherboard. However, to run SLI you must have two nVidia cards in an nVidia chipset motherboard and for crossfire you must have 2 ATI cards (one master and one slave) in an ATI chipset motherboard.
Current Main Rig:
AMD FX55 @ 2.9 GHz
2 GB PC 3200 DDR RAM
ABIT KN8 SLi motherboard
ATI X1900XT O/C to XTX speeds
AMD Athlon XP 2500+ Barton @ 2387 MHz. (217*11)
512MB DDR RAM @ 436 MHz. (PC 3700)
Abit AN7 motherboard
BFG GeForce 6800GT OC!