The AMDPhenom II X4 955 Black Edition is probably the best bang for your buck processor for gaming. The Phenom II X4 955 is a true quad core processor, with a 6 MB L3 cache, and has an unlocked multiplier. This processor can be very easy to overclock and can run as fast as the Phenom II X4 970.
The GeForce GTS 450 graphics card comes in at around $120, and is by far a great card for its price range. You should get a great performance of 16 80×1050 resolution in most games, and 1920×1200 with lower game details. (If you want to get a slightly better graphics cards for a slightly slightly higher prices, consider the other cards in the “Budget Yielding Alternatives” section below)
The ASUS EVO AM3 AMD 790X ATX Motherboard includes 4×240 pin memory slots allowing up to a maximum of 16GB of dual channel DDR3 memory. A built in Audio chipset allowing 8 channels of sound using VIA VT1708S. As well, includes 8 x USB 2.0 connectors, 2 x PCI Express 2.0 x16 (one x16 or dual x8 link), 2 x PCI Express x1, and 2 x PCI slots.
In this build I included a high quality G.Skill 4Gb (2 x 2GB) DDR3 SDRAM 1333 (PC3 10666), a 320 GB Western Digital Caviar Blue 7200RPM, 3.0Gb/s Hard Drive, a SeaSonic 520 Watt Power Supply, a good ASUS CD/DVD burner, and chose the Cooler Master Elite 430 Black Steel / Plastic Computer Case.
All parts have a set of constituent products under “Budget Yielding Alternatives” that you may feel like considering if you want to go with something a little different (especially with computer cases), or better (as in a hard drive with more space, power supply with more wattage for future use, or higher quality RAM).