I recently built a new computer on a budget and it's been running fairly smoothly! The specs are:
Motherboard: Asrock z68 pro 3
Ram: 8GB (cheapest available)
CPU: i5 2500k (OC 4.6Ghz)
GPU: GTX560 Ti Twin Frozr II (OC 950Mhz)
PSU: TX650 Corsair
Case: Zalman 79
Cooling: Arctic Cooling Freezer 13 Heatsink
The Monitor, HDD, Optical Drive, Mouse and keyboard all came from my old build and are fairly old!
This of course, doesn't affect the performance of the machine EXCEPT in the case of the HDD.
LUCKILY, DragonBlogger is hosting an amazing give away at the moment for a Kingston SSDNow V+200! So while I try my luck I suggest anyone interested does the same and importantly, is sure to check out the rest of Dragonblogger.com ! (Giveaway detailed below also)