With such low prices on computer components these days, you can actually get a great 64-bit computer for under $500, including a 19 inch monitor and a copy of Windows Vista.
The Basic Platform
CPU: AMD Athlon 64 X2 7750 ($60)
Motherboard: PC CHIPS AM2+/AM2 NVIDIA GeForce 6100 Micro ATX AMD Motherboard ($45)
While Intel makes some great processors, especially in the high-end, AMD has a much better budget lineup. The Athlon 64 X2 7750 2.7GHz dual core CPU is amazing for the low price of $60. The 7750 actually uses the same chip as AMD's quad-core Phenom processors, but with two of the cores disabled. Clock for clock, the 7750 is better than older Athlon X2s, such as the 5800+. In addition, this CPU can overclock well, with many users regularly hitting in excess of 3.2GHz with the stock cooler.
The PC Chips AM2+ motherboard is pretty standard fare for the low end. Being an AM2+ motherboard, however, means that it can use the Phenom and Phenom II line of processors, and it is also compatible with AM3 CPUs.
RAM and Video Card
RAM: Crucial Ballistix 4GB (2 x 2GB) 240-Pin DDR2 800 ($41)
Even budget builds can afford 4 gigabytes of RAM these days, and this deal for only $41 fits our budget perfectly. 4 gigabytes of RAM is sufficient for any application right now, and for the forseeable future. To fit our motherboard's 2 memory slots, we chose 2 2GB sticks rather than 4 1GB sticks, though it actually ended up cheaper this way.
Video Card: SPARKLE GeForce 7300GT ($26)
Our budget can't afford the best or newest mid-range video cards, but this 7300 GT is still better than most low-end video cards, especially in this price range. This card uses GDDR3, which is a much faster memory technology than is found on most budget cards, and it really provides a nice performance boost. Being a GeForce 7-series card, it doesn't support directX 10, but it can still play nearly any game if you're willing to turn down a few graphics settings.
DVD and Hard Drive
Hard Drive: HITACHI Deskstar P7K500 500GB Hard Drive ($50)
This Hitachi 500GB (half a terabyte) hard drive is an amading deal for $50, as you get 10 gigabytes for every dollar. This amount of storage will be plenty for nearly anyone, unless you're downloading hundreds of HD movies.
DVD Drive: LITE-ON CD/DVD Burner ($24)
This Lite-On DVD burner is a fairly standard model, but it serves our purposes fine.