A awful lot of magazines have benched a 7800gtx against things like sli config of 2 6800 gt's and such and the 7800gtx always come out on top.
The only thing that gets close to a 7800gtx was that dual core 6600 or something like that. The card was massive but scored just a little under the 7800gtx in all tests. I twas ungodly expensive though at the time (probably came down in price), retailed at $600 when it first came out, right after the 7800gtx came out, like within a week. No word on whether or not you could use two of these cads in SLI though.