BleepingComputer.com: Good Budget Build?

Hard Drive: Western Digital Caviar Green WD7500AADS 750GB SATA 3.0Gb/s 3.5" Hard Drive - OEM

Power Supply: OCZ ModXStream Pro OCZ700MXSP 700W ATX12V V2.2 / EPS12V SLI Certified CrossFire Ready 80 PLUS Certified Modular Active PFC

Ram: G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Dual Channel Kit Desktop Memory Model F2-8500CL5D-4GBPK

Case: RAIDMAX SMILODON ATX-612WBP Black 1.0mm SECC Steel ATX Mid Tower Foldout MB Computer Case With 500W Power Supply

Motherboard: ECS BLACK SERIES A790GXM-AD3 AM3 AMD 790GX HDMI ATX AMD Motherboard

CPU: AMD Phenom II X2 550 Black Edition Callisto 3.1GHz Socket AM3 80W Dual-Core Processor Model HDZ550WFGIBOX

Video card: XFX HD-487A-ZHFC Radeon HD 4870 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card
OS: Microsoft Windows Vista Home Premium SP1 32-bit for System Builders

Optical Drive: Sony Optiarc Black 24X DVD+R 8X DVD+RW 12X DVD+R DL 24X DVD-R 6X DVD-RW 48X CD-R 32X CD-RW 2MB Cache SATA DVD Burner with LightScribe LightScribe Support

You're looking at an AM3 system, so you cannot use DDR2 RAM. The PSU doesn't have to be quite as powerful and I suggest a different motherboard. You also should use 64-bit Vista. Here's some of the changes I would make.

Motherboard: ASRock M3A780GXH/128M AM3 AMD 780G HDMI ATX AMD Motherboard - This is a great board for AM3 boards and it can Crossfire if you later choose to do so. $89

RAM: Crucial 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1066 - All AM3 motherboards require DDR3 memory. $60

Power Supply: Here is where you can keep what you have. If you later choose to use Crossfire or get an X2 card the 700W PSU you selected will work well. If you don't plan on using another graphics card, an OCZ ModXStream Pro OCZ600MXSP 600W would be more cost effective.

Operating System: Microsoft Windows Vista Home Premium SP1 64-bit - You need a 64-bit OS to use 4GB or more of RAM. This also comes with a free upgrade coupon to Windows 7. $109