Buliding my first computer. Recomondations?

I want a faster desktop computer, mine does alright but I'd like to play games like Skyrim (Lowest setting is fine) or play GTA IV better. My desktop was built by my dad with the help of a local computer store so if any can be saved to save money that'd be great. I think the hard drive and the OS is only thing save able though. My budget is $400. I was looking into this computer build for $600 it'd be cheaper because of the stuff I have already but still doubt I'd have enough. http://goo.gl/V7KT3 (budget one) but I was wondering if I could get something better. but I don't have much money to spend. $400 is about as much as I can go.

My Current desktop:
Motherboard: Foxconn A6VMX
OS: Windows 7 Home Premium 64 and 32 bit
Processor: AMD Athlon 64 X2 5200+ ~ 2.7GHz
Graphics: ATI Radeon HD 3400 Series
Memory: 2 GB Ram
Power Supply: mastiff dual 550 ~50/60Hz
Hard drive: not Seagate 500 GB (dont want a new one)