The "best" option is this one:
4GB, 800MHz, DDR2 SDRAM Memory, ECC (2 DIMMS) [add $550]
However, that price is absurd. Two 2GB sticks of the absolute best DDR2 can be had for under $150 (that's under $150 total for the 4GB) from any number of computer part retailers. Looks like Dell bought...