DDR/DDR2 difference is important, they are incompatible (if you have other type and mobo supports other).
If you have DDR2 memory and mobo supports DDR2 memory, then they will typically work. Of course there *can* be various problems (as with everything).
For me it seems that you have slower RAM and the mobo you're looking supports up to DDR2 1066 officially. There should be no compatibility problem in this case. It should run with the slower speed of the two, which is 800 in this case. Lower speed simply results in lower performance. I think it depends on the processor if the performance hit will be significant or negligible (at least if you use an intel processor).
(note: i'm not exactly an expert, but i guess better 1 reply than 0)