well I'm using 3x280 (at 750/1614/1150) in my main rig, well at least till there are benchs from the gt300/5870.
the single 4890 has an index of 7, the 3x280 only 6,4 and I doubt that the single 4890 is as fast as the oced 3way ^^
but well, doesn't matter, it's just a poor m$ index which doesn't show the true performance.

I had 2 sapphire 4870 too...even at idle loud as hell and almost 70-80°c.
the 4890 stays quite cool and u don't recognize the fan at all but well doesn't matter, the blocks should arrive on monday =)

quite a busy day today since I'm working on my main rig and the external unit too...changed the fans for some 1200 slipstream and some enermax everest (the 17 kaze 3000 were getting on my nerves ), started to sleeve the pcie cables of the silverstone.
ok the red tubes are looking damn ugly with blue fans, gonna change that too.

the 920 could be a nice one, first test with 20x200 & 1,3v worked flawlessly @ 45-50°c at load.
gonna check max core speed and minimum vcore tomorrow =)