Novice wants to upgrade to gaming rig on a tight budget

I have an older PC my dad gave me with the following specs

-ABIT AN8 SLI w/ AMD Athlon 64 x2 4200+ 2.2ghz stock [S939   ]
-1 gb kit of Kingston Value Ram pc3200/ddr400 cl3 unbuffered (2x512mb)
-Maxtor Diamondmax 300gb SATA 1.5GB/s HDD
-Soundblaster Audigy
-Standard ATX case with one 4 3/4" fan on back and one 3 1/3" in front as well as a tunnel vent thing leading to the cpu fan (not sure how else to describe cooling)  
-Apevia 680w PSU

I am looking to play newer games such as Skyrim, SC 2, Star Wars: Old Republic, and Diablo 3 & Secret World when they are released.

I would like some advice as to how I could use as much of the remaining hardware but upgrade the MoBo/CPU/GPU for as little $$ as possible. I am selling some stuff and hoping to get around $200-$250 for all of it. I know thats very little but maybe you can point me in the right direction on a combo or APU setup that'd be easy for a novice to install in my case.

I was looking to buy a GTS250 to upgrade the GPU and I already ordered the overpriced 2gb kit of PNY DDR400 ram for $35 and a wireless-g pci adapter but thinking of returning the ram to add my cache of funds and just overhaul the entire thing. Thanks in advance for any advice you can give me.