Don't get either of them. The best processor for your needs is the "Woodcrest" Intel Xeon chip, which currently maxes out at 3Ghz and is often found in a dual processor (quad-core) configuration. These are the latest-greatest from Intel and are significantly better than either of the chips you're looking at, even though the clock speed is lower.
EDIT: I was tempted to tell you to buy a
Mac Pro from Apple and just run Windows on it, since it's the cheapest way (in the ballpark of $1k cheaper) to get quad-core Xeons under your desk. Your graphics card needs might rule that out, though.
