So what kind of aggregate transistor count, memory count, and memory bandwidth are we looking at there across four HD5870's? Not going to see that kind of performance from a single GPU for another four years or so.
True, but I imagine they are loosing quite a bit of the theoretical performance because of inefficiencies involved in multi-gpu scaling. Not to mention other improvements in technology over a four year span. IIRC, fours years ago G7x was NVIDIA's currnet gpu. If you were building a system today, would you rather have a 7950GX2 Quad SLI rig or one with a single 5870?
edit: Your post actually got me wondering how 7-series Quad SLI would compare to a single 5870. While it's not exactly easy to find an even remotely direct comparison because the games benched and cpu/motherboard hardware has changed over time, I did find some 8xAA Quad SLI Oblivion benchmarks and some 8xAA Fallout 3 5870 benchmarks. IIRC, these games use the same engine.
7-Series quad SLI Oblivion (Mountains) @ 1920x1200 8xAA/16aAF: ~47fps
http://www.firingsquad.com/hardware/geforce_7900_7950_gx2_quad_sli_update/page9.asp
7-Series quad SLI Oblivion (foliage) @ 1920x1200 8xAA/16aAF: ~30fps
http://www.firingsquad.com/hardware/geforce_7900_7950_gx2_quad_sli_update/page10.asp
Radeon 5870 Fallout 3 @ 1920x1200 8xAA/16aAF: 71fps
http://www.techspot.com/review/198-ati-radeon-hd-5870-review/page7.html
The quad SLI benchmarks were run with bloom as well because G7x didn't support HDR + AA in Oblivion, while Fallout 3 was benched with HDR.
Amazing how far we've come since 2006. 7950GX2 quad SLI would have been a $1100-1200 video card setup, while even a priced gouged 5870 sells for $400-450. Definitely puts the video card into perspective. I've always been a proponent of spending enough to get what you really want, but these multi-gpu "drool boxes" really show how outclassed your investment can be in a few years.