go with 2 x 2gb of RAM if you can, make use of dual channel.
Also, there’s absolutely no reason to not at least get a DVD-Rom, considering one can be had for $20-30. If you might possibly watch blu-rays, you can get one for a modest $60-80
Agree what aaron says, exactly your budget will determine any suggestions–mainly on the CPU & motherboard. Obviously if you could splurge a bit on a Sandy Bridge solution you would be extremely happy, but your wallet might not be
