I've heard nothing but good things regarding the sapphire cards. XFX on the other had can be a bit problematic...it seems the newer cards haven't been getting the quality assurance checks that they should be getting. I've seen several reviews on amazon and new egg about the fan failing on the XFX. The older cards don't have the same problem. The fan on the XFX is rather loud, I've been tempted to get artic coolers for it, but still deciding between water cooling or air cooling.
Also, if you do go with the XFX, make sure to get the older model that only has one dvi port. When they switched to the dual dvi ports, they changed the PCB to revision 3.1. In that revision, they locked the ability to change the voltages. I don't believe it is even possible to work around that with a bios flash.
The best that I can do for overclocking on the stock cooler setup is 1240 clock and 1650 memory. Keep in mind that is in crossfire setup which can lower overclocking abilitiy. I don't know how far I could push it in single card setup, never tried.
Also, crossfire does induce the possibilty of micro-stutter. I've heard that Radeon pro tools helps address this issue, but the only other way to reduce micro-stutter in a multi-gpu setup is to go tri-fire. I will say that micro-stutter is somewhat subjective...some people notice it more than others. Unfortunatly, your board won't support a tri-fire setup.