HD 4890 vs Crossfire HD 4890\4850

I have a simple problem -- I can't decide whether or not to drop my old HD4850 (1gb DDR3) into my gaming rig in crossfire with my recently purchased SAPHIRE HD4890 Vapor-X (1gb DDR5). I'm getting excellent performance from the 4890 on its own, but the geek in me is greedy and wonders if I can squeeze out a little more performance. My gaming rig has an i7 (stock), 6gb DDR3, plenty of PCI lanes on the board, and a beautiful recently purchased ANTEC 750w power supply. My understanding is that the 4890 will scale back to the speed of the slower card, basically making a 4890/4850 crossfire equal to a standard 4850/4850 crossfire. I've started this thread because I've been unable to find any solid information or benches comparing these two setups (ie. 4890 vs 4890/4850). Also, while there is some information comparing a 4850/4850 crossfire with a sole 4890, I have yet to see a single benchmark for a pair of 4850's with 1gb DDR3 (as mine has) as opposed to the standard 512mb most 4850's have.

So is it worth the time and effort to install the old 4850? Anyone who has seen benches for 4890/4850 crossfire or 4850/4850 (1gb DDR3) please post here.

Side note: recomendations for a free benchmarking tool would be apreciated (PCMark wants to get paid and I have geek greed)

Thanks!