Spent about $550 two years ago to create this:
CPU: AMD Athlon 64 X2 BE-2400 ($25 steal, figured I could always upgrade later if I needed to. Haven't needed to!)
Mobo: ASUS M3N78 PRO
GPU: NVIDIA GeForce 8800 GT 512MB
HDD: 1x500GB SATA
RAM: 2X2GB PC2-6400 G.Skill RAM
PSU: Antec average cost PSU.
CD/DVD: Just your average DVD-RW
Cooling: Stock
Overclocking: None
Windows XP Pro
And just finished putting together the wife's PC in this:
For around $400 out of pocket, she has:
Mobo & CPU combo - Biostar. It's not amazing, but the most graphically taxing single player game she plays is Sims 3, so I'm not worried.
GPU: Sapphire HD 2600 XT (a very generous friend donated this.)
HDD: 1x750GB SATA (aforementioned generous friend also donated this!)
RAM: G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800
PSU: Antec ECO 620C
Windows 7 Pro