Maybe a sound card? For normal use the onboard sound will be fine but for gaming you might want something more like Creative's X-fi. I think those harddrives are overrated. Just get some nice SATA drives. Both Seagate and Maxtor have some nice ones with 16MB cache atm. Although having two harddrives is nice. I mainly use them for backing up data I would be screwed without (like my PT db's), so maybe getting one raptor and one large drive would be a good idea. Raid 0 is overrated just for the mere fact that it doubles your risk to drive failure.
You may also want to get an aftermarket CPU cooler and some nice quiet fans for the case. Thermalright makes nice cpu coolers (both the XP-90 and XP-120 are great). I hate loud computers. It wasn't much of a problem when I was only gaming with headphones on but playing poker I would hate having a loud computer now. A good brand of fan when looking for something quiet is Nexus. Both
Jab-Tech and
End PC Noise sell them. Don't go by the dB specs most manufacturers give you though. They are almost always wrong or lies.