alrockey:
having gone through this issue, recently, I'd suggest you go with PC3200 DDR400; you have a processor that passes data at quad your memory path, you want the fastest memory that works. Dell/Intel's bios is tamed for stability (afterall, this is a server). Plus, they haven't provided cooling to tolerate overtweaking and therefore no choices for it in the BIOS. Finally, the cost of said modules are within a gentle squeeze of PC2700. As Olliebg states, his 1G of Crucial operates his machin-o nicely. I doubt that he'll not keep his system.
I also decided to go with Crucial's 3200 despite its CL3; unless you can overclock this system, you'll never need faster memory. And, given the quietness/and/or absence of fan buzz, I wouldn't ever sacrifice that for tweaking more speed out of my memory; my choice.
Finally, if you opt for 512M as an initial complement of RAM, decide on complementary RAM that won't negatively effect your Dual Channel performance. Crucial, afterall is Micron, (although they produced their first PC3200 with Samsung chips) and if you order modules subsequently, your chance of getting them with similar characteristics is very high. Those issues all contribute to good Dual Channel performance, if you're not going to do a total population of your memory slots at once.