oops...sorry...I really screwed up that post. I hope you can understand it. It was the best bang for the buck I could come up with. Here are some options to consider.
You could upgrade the MB to the ddr3 version for another $30
DDR3 memory would be slightly more, so add another $20
I also did not include an aftermarket cooler, so you could add another $50 for that for extreme OCing.
You could get by with a smaller power supply, but I assume the first upgrade you make would be to add a second 4850, one 4850 is fine in most games at up to 16x10 resolution, but if you go to 19x12 resolution you will need to add the second 4850. ..and for future upgrades you'll want to have at least a 750w power supply.
You could save some on the processor by going to the dual core 550BE, but I'm not sure the tradeoffs there are worth it.
Bottom line is, for $800 (ddr2) - $850(ddr3) you will have a good base system. You can slowly upgrade and put another $150-$200 (aftermarket cooler for higher overclock and second video card) in it and have a really high end gaming machine.