XFX is one of the few ATI card companies that supports overclocking in their warranty therefore it would be a good option because you could overclock it to get those extra couple frames out of it and if anything goes wrong, you're still covered.
I just got an XOC 4890 from Diamond Multimedia last Tuesday. I'm running it at core clock-950MHZ and Memory at 1050Mhz. No issues so far. I got it for $229. I plan to crossfire with exact the same board soon.
I have use Diamond more than once with OC version. No problem with their boards.
I have use Sapphire before but did not overclock the board.
+1, XFX is a pretty awesome brand, but I sorta like Sparkle Nvidia cards, I don't know why but certainly they're not one of the BIG manufacturers like EVGA, BFG or XFX. They have pretty good customer service though.