I thought the 8800's were faster than the 3870's? I contemplated getting one, but with 10.1 (that may never be used) and they scale better in X-fire and overclocking.
Perhaps you will gain with a system cleaning and overclocking...
On your board/memory issue, do you have cool and quiet enabled?
I managed to get my CPU up a few more Mhz, by dropping the vcore to 1.38. It was hitting 54C running the SMP of F@H, now it runs 47C @ 2.7Ghz
Next up for me is a full system upgrade.
2 SAS 15K disks on a dedicated LSI PCI-e card
1 750Gb Storage drive
Dual boot Vista/XP
Either a phenom if the fix they implemented works and the new 45nm clocks better, or a quad core Intel.
Another 3870 for X-fire.
Keeping my current 3870, sound card, Tuner card, and peripherals.