Actually, according to a certain unmentionable website, 7800 gtx 256 sli beats the 7900 gt sli in quite a few tests and is pretty much on par with the rest. I also have 7800 gtx 256 sli and can play any game under the sun.
I bought my BFG cards when they first came out so I paid quite a bit for them. I would guess, and this is only a guess, that they run about 200/250 per card these days if you can find them.
Just did a quick check at Newegg they have 1 manufacturer new at 349 and 1 recertified at 249 so it looks like my guess was fairly accurate. Good luck and you will definitely enjoy these cards and BFG has lifetime warranty.
I would personally go with 7900GT's in SLI just for the simple fact that they overclock to 7900GTX speeds with very little effort. and they can be found for fairly cheap. check sig for my overclocks