How does performance of SLI Nivida 8800 GTX compare to duel ATI 5670s?

The GTS 250 (rebranding of the 9800 GTX+ which is an OC'd die-shrunk version of the 8800 GTX 512MB I believe) is comparable in performance to the 4850/5750. But dual 8800s would be a LOT hotter and more power-hungry.

So do you think I should accept new system? Use it mainly to play games and don't want to see any decrease in frame rate, etc.

There was no 8800GTX 512MB, I think you are getting things confused with the G80 8800GTXS 640MB and the 8800GTX 768MB.

i wouldnt do it, 2x 5670s is weaker than 2x 8800 GTX 768MB.

the 640MB version was a 8800GTS btw  

So it was, I stand corrected.

I'd say that is a ripoff. 5670 CF looks pretty dismal to me. Here it is losing to a single GTS250 1GB which can trace its roots back to an 8800:

http://www.guru3d.com/article/radeon-hd-5670-review-tes...

I agree mate. A pair of 8800GTX's would still be usable gaming rig today and far better than a pair of low to mid range ATi cards IMHO.

Could you list the full old and new system specs?

See if you can convince them to give you at least a pair of GTS 250s or something. Does it have to be crossfire?

No way that 2 5670's are better or equal to 2x 8800gts in sli, they are trying to r0ot you in the A**hole. I would accept a sli gts250 1gb setup as a valid replacement. If needed to go the ati road 2x 5750's would be more equivelant. OR ask them to bump you up to a better single card if possible. a 5850 or 5870 or gtx470 would probably be equivelant. Then you can always add a second card yourself for even more power. Heres some benchmarks for you http://www.tomshardware.com/charts/gaming-graphics-card...
http://www.tomshardware.com/charts/2010-gaming-graphics...

There is nothing i can find comparing those cards directly, but you can see the gts250 sli is very similar to 8800gtx sli, in the second link you can see gts250 sli beats 5670 xfire most of the time

Thanks, guys, I can't thank you enough.

Sure you can, send beer not hair-dryers.