Here's my 2 pennies worth
AMD A-10 6800K £109.99
*I know it isn't mATX but it's worth considering* Gigabyte G1 Sniper A88X FM2+ (loads of features, upgradeable in the future to Kaveri) £89.99
Kingston Hyper X Beast 8GB 2133MHz £71.99
Kingston 120GB SSDNow V300 (enough to get you started, you can add more storage when you can afford it or use an old drive) £67.99
Corsair Hydro H55 £52.99
Bitfenix Outlaw Gaming Case (I like the white and red version) £44.99
Arctic Silver 5 Thermal Compound £8.99
Corsair CX430 80+ power supply (plenty extra to add a Radeon 7770, extra drives, etc, in the future) £43.99
Total cost in GBP £490.92. A great budget PC with lots of upgradeability.
These are U.K prices sourced from www.overclockers.co.uk You might be able to acquire the items cheaper elsewhere especially in the U.S