I would avoid the Xeon 50xx processors. They're older, slower, and hotter, and more power hungry than almost any of the Xeon 51xx processors (50xx are Netburst based, 51xx are Core based). For just a tiny bit more money, you could get a Xeon 5130 which should be comparable or better than the fastest of the 50xx processors.
As for RAM, you could possibly save some cash by getting 4x1GB instead of 2x4GB. Depends on your plans for future expansion and how many memory slots you have. I know most databases love more memory, so give this some careful thought as to your expansion needs vs how much you want to spend.