Btw, if you want to reuse some of your components, you can reuse your DVD drive and hard drive. You can reuse your PSU, but I suggest getting the new one I listed above since prebuilt PCs usually have weak PSUs.
You can reuse your DDR2 if you pick out an AM3 motherboard that supports DDR2, which will save you another hundred dollars or so.
Of course, you can also just sell your DDR2 and other parts on ebay, etc