Example $450 build:
http://ca.pcpartpicker.com/p/dcMYQ7
1) I recommend adding an identical memory stick later on to get up to 8GB. I didn't go with 2x2GB because there were only two slots.
2) If I drop below a GTX750 graphics card, the performance drops off very quickly. In particular, look at the HD7750 and GT640 relative to the GTX750Ti:
http://www.techpowerup.com/reviews/ASUS/GTX_750_Ti_OC/25.html
A GT640 costs about $90 and gets 35% the performance of the 750Ti that costs $150.
*For reference, the GT640 when paired with the A10-5800K (replacing the graphics part) got about 50% improvement so the GTX750 + X4-750K might be as much as 3X the performance for about $50 more for the build!
To repeat, the X4-750K + GTX750 system, would probably average about 3X the frame rate as the A10-5800K setup. (Unfortunately I can't find everything on one benchmark)