What is the cheapest way to upgrade this system and improve its gaming performance? Note that I will not upgrade this for a while yet so the question is more about what the weakest part that needs upgrading is.
None of these components are overclocked.
CM Storm Scout Midi Tower
Chieftec Smart Series 550W PSU
AMD Phenom II X4 945
MSI 770-C45, Socket-AM3, DDR3
Corsair XMS3 DDR3 1333MHz 4GB CL9
Western Digital Caviar® GreenPower™ 1TB
XFX Radeon HD 5770 1GB GDDR5 875mhz
BenQ 24" LCD G2420HD 1900-1080
Windows Home Premium 7 64bit
None of these components are overclocked.
CM Storm Scout Midi Tower
Chieftec Smart Series 550W PSU
AMD Phenom II X4 945
MSI 770-C45, Socket-AM3, DDR3
Corsair XMS3 DDR3 1333MHz 4GB CL9
Western Digital Caviar® GreenPower™ 1TB
XFX Radeon HD 5770 1GB GDDR5 875mhz
BenQ 24" LCD G2420HD 1900-1080
Windows Home Premium 7 64bit