The specs looks fine to me, with the exception of the PSU. I don't think 300W will be enough for one of those 6800GT monsters, you're gonna need a new one. Apparently, and I'm no expert on this, you should have at least 24 amps on your 12V rail; you should find these specs when you look at PSUs on various retail websites. As long as you get a decent brand like Antec that is about 400W or more, you should be ok as these ought to have enough current on the 12V rail by virtue of being good PSUs.
There are plenty of people here that know an awful lot more about PSUs than I do, hopefully someone will post about it. You could also try looking through the forums for other posts about PSUs.