To upgrade to ddr3 you will need a new motherboard.
8GB of ddr2, probably not cheaper than that. 8GB is alot of ddr2.
If by donor, you mean build a new pc but use some of the parts from your current desktop, then yes.
Most likely you can use everything except the motherboard. You may need a new CPU if its not recent enough, but if you are getting a new motherboard anyway, might as well get a faster cpu. Plus you can usually get a good deal on a motherboard/cpu combo.
I guess before we get too far into this, what are you looking to accomplish with an upgrade? The reason I ask is that dram is not typically the bottleneck in a system. The hdd is the main bottleneck and an upgrade to an ssd would seem like getting a new pc.