Realtek works, just make sure it's different to the onboard one, or you may suffer from the PCI ID overlap issue (although this can be fixed with some jiggery pokery, it was just beyond me personally). I bought a really ancient 10/100 one, and use that for the external network, and use the onboard gigabit one for the internal network.
Also I don't think all the RAM can be utilised, think max is 3 GB. I would buy a matched (2 x 1 GB) pair, see if that meets your needs, else upgrade with another 1 GB stick at a later date.
Cheers,
Matt.