Hmm... could you handle around $275?
http://pcpartpicker.com/p/1g5l5
AMD FX-6300 3.5GHz 6-Core Processor
ASRock 970 Extreme3 R2.0 ATX AM3+ Motherboard
Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory
Base Total: $274.96
Promo Discounts: -$10.00
Shipping: $7.56
Total: $272.52
Because anything less wouldn't be much of an upgrade. Unless you overclocked some 750K Athlon Processor to like 4GHz or something but then you'd simply have an overclocked version of your current system.