Title explains it all
go on newegg
find a 955be or something similar thats on sale.
find the cheapest am2 mobo
load that bitch up with the cheapest ram you can get
find a case thats cheap and comes with a ~400 watt psu.
spend the rest of your budget on a graphics card.
take the hdd and psu out of your current computer. now check to see what psu has higher wattage (the new one or your old one) use that psu for your computer, but not the gpu. now take a paper clip and make a jumper between the green and black wire on the spare psu. hook that shit up to your gpu.
congrats. you have a very gettoh computer, that can now max out basicaly any game for the next few years.
Build I put together a few days ago.
http://pcpartpicker.com/p/lggd
Should be able to OC the CPU to about 4.7GHz with this build. Excellent overall performance. $653 after rebates.
I would go with cloudscorpion's build
I would probably go for an i3 and get a $200 graphics card if you want as much performance for gaming.
I've already made one and compared benchmarks for this price point the 6300 FX is the way to go