I am planning on getting a custom computer built (yeah I could try to build it myself, but my luck is not the greatest and I would probably screw something up anyway). I would like some help in figuring out what kind of motherboard I should get for a computer that has:

AMD Phenom II X3 720 Black Edition
ATI Radeon HD 4890 1GB

anything else I should also look into? Like powersupply?

You have to ask some specialist in computer hardware. You can go to multi-able electronics stores and collect the different ideas and thoughts, also the offers they have. .