Asus P5K Pro is a p35 mobo, you would only be able to run your 4870 at PCIe 1.0 16x which is equivalent to PCIe 2.0 8x speed. Benches have shown about a 5 to 10% decrease in performance when running not at the full 2.0 16x speed.
If you care about that and dont plan on Crossfire then you could go with a P45 or P43, the P43 boards can be found for around $100 now the same price as your Asus P5K Pro.
If you do plan on CF then a x38 or x48 would allow you to run 2 slots at PCIe 2.0 speed, but you would spend around $200 for that.
If you wanted to overlock I would suggest geting an aftermarket cooler.
your build looks good though, have fun.