I have the same board. I am running a plain ol' Barton XP2500+ at XP3200+ speed.
Can you change the mulitplier on that chip with your mainboard?
According to the manual, the A7V880 will support Dual Channel with all 4 slots filled. Caveats apply. As JohnDoh pointed out, more stix, more issues. Horse trade until you get a good matching set of decent DIMMS.
I am dying to know if this board will allow the XP Mobile to change multipliers.