In case you haven't settled on a processor yet, don't get a quad. The primary function of the system will be gaming, and a quad won't give you anything applicable to gaming. As for memory, 2GB sticks are pretty cheap these days. Depending on the specifics of your final budget, 2GB may be the best option. Getting one 2GB module that is common will let you wait a few months, then you can easily find the same module and add another (or even 3). Don't start with more than a pair of 2GB sticks though.. I doubt more will help gaming performance.
For the hard drive, I'm against velociraptors. Those are basically the hard drive equivalent of dyed water in a water cooling setup, or speed tape on the brake lights of your car. If you really want speed, get a slightly more expensive motherboard with SAS on it then get a 147GB 15K SAS drive. Half as much space, yeah, but way the hell faster, more reliable, and cheaper too. - I can't remember/find the board I'm thinking of, but I saw one a few days ago that was around $200 that had SAS. Even the cheapest one on newegg ($270, plus $120 for the drive) will still be about the same cost as a non-SAS board plus the raptor.
If you want a few other complete systems for comparison, see arstechnica's
system guides.