Search around the forums, this question hss been asked many times.
-Dual Channel RAM access (when using 2 identical DIMM's)
-The new line of Athlon 64 X2 (Dual Core) CPU's, thus it's futureproof
So, you really want to get the 3000+ if you want to upgrade your CPU in the future.
AMD X2 4400+ Toledo 1MB/Asus A8N-SLI Deluxe/2GB Kingston HyperX/SLI Gigabyte 7800 GTX 256MB/Creative SB Audigy 2 ZS Plat w/ Creative Inspire 6.1 speakers /180GB Hitachi PATA/19\" AG Neovo F-419 LCD/OCZ PowerStream 600W PSU/WinXP Home SP2