Well, RAM is cheap enough now that I think it's worth it to go for the newer/better chipset rather than re-use the DDR2 you've got. Unless you are on an extreme budget, that is.
Of course, DDR2 on a 785G is perfectly fine for HTPC use (that's my setup, BTW) and the performance difference between DDR2 and DDR3 is not huge. You do get a bit of power savings going with DDR3, and that's always a plus.
Anyway, I say go for the newer 880G board.
That's my 2¢.