well my friend, the performance will be oustandingly better with 2x 6800gt's, however, image quality will not be as good as the 7800gt. I'd personally get the duel 6800gt, that'd be KICK ASS, you'll be able to run bf2 with no troubles with either 7800 or 6800 though.
hows ur budget looking? it all depends on if your choice of either buying a new mobo and also purchasing these v-cards, or you can just keep ur mobo you have now and buy a card for that.. so tell us, what do you want.. new sli mobo.. we're looking at 150 - 250 bux.. let me know, then i'll help you choose, MOBO OR NOT TO MOBO!