I built mine on a budget, trying to keep under $500.

CPU: AMD Athlon II X 4 @ 2.6ghz
MB: ASUS M4A785TD-M
RAM: 4GB Corsair DDR3 @ 1333mhz
PSU: Rosewill RP600V2-S-SL 600W
GPU: Radeon HD 5570 (I thought it had HDMI when I bought it, stupid me. I have to use a display port to HDMI converter.)
HDD: 250GB Seagate 7200 RPM
Case: NZXT GAMMA Classic Series GAMA-001BK Black Steel ATX Mid Tower Computer Case
DVD: Can't remember. Lite-on? Sony? Not that it matters much.
OS: Windows XP SP3. Yes, I know about the memory limitation. I need to upgrade to W7.

I play fullscreen on a 42" Vizio at 1920x1080, with the background drawing distance set to high, normal drawing set to standard, and the textures set to standard. I can turn these up to high but I start chopping a little. I prefer a smooth framerate to excellent visuals.

Finally, for input I use a PS3 DualShock 3, and a Logitech K350 wireless ergonomic keyboard and mouse.