Most coolers for AMD systems use the bracket built into the motherboard to secure the heatsink. If you want a really nice cooler that mounts this way the Scythe Infinity will suit. If you want something less massive and expensive the Freezer 64 also uses a simple clip in installation.
Two 7800s is nothing to scoff at, and the GTX will stress pretty much any cpu out there, but if you really want DX10 support then you could go for it.
If you do not want to remove the mobo, you do not want to break your wallet, want a silent cooler and a really good one, try the coolermaster hyper tx. It uses the retention mechanism of the socket 939. The only thing you check is
1. It can fit in your case
2. Make sure you ask for the amd version
I agree. I'm using one on an Opteron 180 @2.52Ghz, which keeps tempertures 26-30ºC, depending on whether the central heating is on or not
They're supposed to be pretty quiet too, although I can't tell as the stock cooler on my 1900XT and 5 case fans are louder