I hope I don't come by as insulting, but putting computers together yourself is basically foolproof if someone has already checked your parts. The "planned obsolescence" (nothing planned as far as processors go. Things just move so fast they outstrip the interface) works in your favor, as you can't really plug anything in where it won't fit. With mine, I had more physical problems than anything else, like getting everything inside the case, reaching the main power from the bottom mounted PSU to the motherboard, etc.
Some general stuff-
If you want ~100$ for a processor, i3 all the way. I have a AMD Athlon II x4, but it can actually be beat or matched on quite a few quad+ threaded benchmarks by the i3 at stock, not even speaking of OCing or single and dual threaded crap.
For the card,
Tom's has the best reference, really. I would recommend going up to GTX460 or the such. It's "only" 150$, and will run just about anything on high.
PSU - Building your own, or upgrading a premade, you will need to replace this. Premades tend to have PSU's barely able to handle the system. Look for something with good ratings on newegg, and read the systems people have running on them. 600w+ with ~40+ amps on the 12v rail(s) is where to start. Reviews are your friends here, nothing is really to the same measurement.
Motherboard - pick features, price range, choose one with best reviews. Pretty simple. Try and look for USB3.0 support. Make sure it has PCI-e 16x 2.0 or newer. Apparently there's a 2.1 spec or something, CBA to look, but it will be compat with 2.0. 4-6 ram slots is nice. **DDR3 compat is a must**
RAM - Once again, reviews on newegg. 4GB bare minimum for a gaming rig. Considering I've seen ~400$ premade computers with 5 gigs (no clue why 5) of DDR3, this is pretty easy to get. DDR3 compat is a must, why? Cheaper, faster. DDR2 is falling out of production, so it will be much more expensive soon.