Spend a little more and get a much better system.
If you want to stay with AMD, go for an AM3, AM3+, or FM1 motherboard.
AMD has good quad core processor that start at under $100.
All new CPUs use DDR3, and luckily that's pretty cheap (cheaper then the outdated memory you'd have on a Socket 939).
If you go with an FM1 board, with the right processor you won't need a video card initially.
I did a similar upgrade to this myself a few months ago.
I bought a Phenom II X4 955 processor which easily overclocked to 3.6 GHz for $99 (http://www.newegg.com/Product/Product.aspx?Item=N82E16819113007)
My Asrock motherboard was $69.95 (if I was in the market today I'd buy this - http://www.newegg.com/Product/Product.aspx?Item=N82E16813157292).
4GB of DDR3 - $24.99 (http://www.newegg.com/Product/Product.aspx?Item=N82E16820145278)
You'll need a CPU cooler (I had one), buyt you're still talking around $200.