Are you talking SLI?, 'cause there's no way a single card is gonna draw 22 amps.
I know for a fact the BFG 6800GS OC AGP draws 18A, because I have it. I wouldn't be surprised if a single 7800GTX 512MB can draw 22A.
Anyways, you can try
to run it on that PSU, but I wouldn't recommend it. It might run for awhile, and then burn out. If I were you I'd get a safer, more stable PSU (you can get a good PSU for as low as $40) and stay on the safe side rather than risk frying your video card.
Those BFG lifetime warranties sure can come in handy though, can't they?