Hardware Tips: For Dummies

The most popular processor on the market is the Intel Q6600. It is the cheapest quad core they make, and it very, VERY easily overclocks to 3.0 ghz and beyond. Pricing has dipped below $200, but is at around $220 right now.

If you overclock that chip, you want an aftermarket heatsink an fan for it, let me recommend the CNPS 9700 by Zalman ~$50

With your budget, you want DDR2 RAM because it is vastly cheaper than the newer, more expensive DDR3. You probably want 4gb of DDR2-800 (800 megahertz speed) for under $100.

SLI (Scalable Link Interface) is where you bridge 2 (or more) graphics processors into 1 output. Most multiboxers want the exact opposite, 2 outputs from 1 card, so they can use dual (or quad) monitors. DO NOT SLI FOR WOW!!! Crysis yes, but this is a multibox forum. I still like the 8800GTS. See Toms Hardware for some in depth info (on any of these topics)

Motherboard: Here is where you can really show what you know (or don't). The above mentioned Q6600 is a Socket775 processor, as are most other Intel processors out there. This should be your first specification. Next important is: onboard Wifi (or not), PCI Express 2.0 (vs 1.0), raid controller, onboard audio (much better recently than in years past) and size (microATX, mid-tower, fullATX).
Beyond this, a true enthusiast would know exactly which chipset he wants: NVidia for SLI compliant systems, Intel for Crossfire or non-SLI. My most recent build has an x38 chipset :P

My best advice, read Toms Hardware for a month, before making any decisions. It will learn you good.