There are very few motherboards available that can use both DDR2 and DDR3.
I put my beaters on this Gigabyte board at my local MicroCenter 2 weeks ago...it has support for both:Gigabyte GA-P35C-DS3R
Do you have a motherboard yet? We can probably tell you what it uses.
As far as power supplies go - you shouldn't have any trouble....unless it's 1KW+. Some of the power supplies in the 750w range can also be a pain to install - but fit none-the-less.
Continue to post what you're shopping around for - there are some serious rig builders around here. I know they won't mind helping out.