SLI/CF Never scales 100%
Ie: If u get 60fps with one card u will prob only get 100fps with 2 cards.
Thats why it never makes sense going sli/cf UNLESS ur at the very peak of performance, like 2x8800 gtx. People wanting 2x7600's are just wasting their time.
As for 2x2900's. Given the massive price increase over a single gtx id HOPE they would be faster, but id still prefer 1 gtx.
I had a 1950xt (on water, which DID have a leak which i didnt see for ~24 hours till green fluid started leaking out the bottom of the case. Ran down from the cpu, down the mobo, pooled on the card, down the mobo, thru the pci slots. Didnt kill anything the )... and that ran quite hot, so i can believe 2x2900's being close to each other will run quite hot.