If you can live with an mATX board then I'd take that, its the cheapest. For full size board, the UD3R is $15 cheaper than the UD3P because it doesn't support crossfire, which for you doesn't matter as you've chosen a nvidia card.
A good 400W is all you need, 450W is what the manufactuer recommends, 550W could give you the room you need for a future power hungry card, but who knows what the power requirements will be when you choose your next video card. Either option above is good, the OCZ is a better "brand" but the reviews on the Tuniq 550W were good.
You can upgrade to to a Phenom II 810 for $35 or the Phenom II 940 $85.
Thats leaves whatever you want to spend on a GPU.
You can go with a 4830/4850/4870 to leave the option for a xfire upgrade path down the road, or a GTX 260 216 core for $170.
Given you budget this AMD build will give a significant performance advantage mainly because the cost/performance of the CPU/motherboard/RAM which frees up money to dump into the GPU.