I built my own gaming / office PC for the very first time a few months ago. If you buy good stuff now you can keep upgrading for years and years. I did mine on a $750 budget and came in a $740. Most bought at TigerDirect.com.
Case- ThermalCooler, forgot the number, $59. Came with three 120MM Fans.
Motherboard- MSI Z77 / 1155 - bunch of companies make this, $124.
CPU- Intel i5 / 3570k - Can overclock this one up to 4.7Ghz if need be, $199. I run mine at 4.2Ghz.
GPU- Sapphire HD6870 - I like AMD graphic cards, $100 used. Just upgraded this to Sapphire 7970 (R9 280x).
PSU- Corsair 750W - Don't skimp here, 750W is the minimum, $75.
RAM- 4G is fine now days, I run 8G as a minimum, it's cheap $59.
HD- Western Digital 500G, $65. A Solid State Drive (SSD) is even better if in your budget.
CD-ROM- Bought a basic LG / DVD unit for $29. It's a gamer not a movie machine.
Accessories- 240MM exhaust fan for the top, 120MM side exhaust fan (case came with 3), $20.
OS- Already had Microsoft Windows 7 / 64 bit. Great for gaming, work, home, etc, $0.
Total- $740
I run triple 24" HD Monitors with all this.
The Z77 Motherboard comes with 7.1 RealTek Surround Sound, SLI is you want to run 2 GPU cards together, comes with an OverClock genie app (can even do it on your smart phone) and more. Very important to get a good main board.