Mix different speeds no problem, system will just run both at the lower speed,
AM3 board for ddr3 will only take ddr3 but I found this whilst proposing a build for a client,http://www.overclock.co.uk/product/AMD-Athlon-X2-245-2.9GHz-CPU---2GB-DDR2-800MHz-RAM---nForce-630a-Motherboard-Bundle_20604.html
if you read down the page for specs,
The mobo supports both am2 and am3 cpu's (no news there),
but it also allows ddr2 or ddr3 memory depending on which chip you have, so if your on am2, its ddr2, and I assume am3 will only run the ddr3
but if one board has that capability, I'm sure there will be other instances of it as well,
could you consider a new build with ddr3 and maybe keeping the old ddr2 rig for other usage?
Folding/ seti research/ 'private' browsing