There's 3 product lines that you need to look at.
1 - The Q6xxx - The mass market desktop part.
2 - The X32xx - The rebranded desktop part.
3 - The E53xx - The actual server part.
Which one you chose depends on the intended use, cost, performance and reliability.
Number 1 is your cheapest option, and will no doubt get you the best price/performance due to rock bottom prices. You've got a cheap CPU, a huge selection of cheap motherboards, and rock-bottom DDR2 prices. That's without mentioning the free upgrade you could get by overclocking if you know what you're doing.
Number 2 is an option for the uninformed tech guy looking to save a few quid..."Look you can get this Xeon for cheaper than that one!"
Number 3 is the option for a company that puts reliability above everything else. Price shouldn't be a deciding factor, and the server should be intended for a serious purpose. Server motherboards, FB-DIMM RAM and S771 (same CPU as the Q6xxx but different socket) CPUs are very expensive.
My guess, as you're posting on here (no offense) is that you're looking for #1. You'll be able to spec a "Quad Core, 4GB RAM server" for a price that your boss will love - just make sure you don't scrape the bottom of the barrell with components such as the motherboard, otherwise you'll be explaining why it has to be rebooted every couple of days. Save money, but not to the factor of a couple of quid here and there.
HTH
