Overall, you have the right idea but it could use some changes.
Instead of selecting the most expensive AM3 board you will be getting the same performance out of either Gigabyte's UD5P or MSI's GD70 boards. If you can find Mushkin RAM at lower timings in 1333MHz you mine aswell get a pair of those instead. The PII's only recognize up to 1333MHz bandwidth. I didn't know that until I actually posted and showed up to 1333MHz after switching to 1600MHz.
No need to get the 4870. Just get the 5770 instead. It runs DX11, stays cooler, and uses less power. You won't need 850W or power to run your system either. With 5770's in Crossfire you can still get by with a 650W 80+ power supply. The Caviar Blue's aren't exactly the best bang for the buck. Get the Samsung Spinpoint F3 instead. Should be cheaper if not in the same ball park.