Your old motherboard uses a socket 939 for the CPU. None of the new AMD CPU's will work in that board. The board also uses the old DDR1-400 memory modules which also won't work with any of the modern boards. As you guessed, your looking at MB+CPU+RAM.
MB -
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128521$80 Free Shipping, $70 after MIR
CPU -
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103727$100 free shipping. If your a newegg subscriber you can use code EMCNAHB29 and get it for $90. retail box so Fan included.
memory -
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231428$35 free shipping using promo code EMCNAHE42.
Then of course you have your video card.. The 6850/6870 are good choices.
The CPU above is the older Phenom IIX4 line, and it actually performs better in games than many of the newer FX line CPU's. The Phenom IIX4's have 6MB of L3 cache which makes a huge difference. My second PC uses this CPU (main is an i5 3750K), and it runs Guild Wars 2 really well with an older radeon 4870 video card. Toss that 6850/6870 in the mix and he should be in seventh heaven.
