I recommend you save up money and do a new build. A cheap AMD CPU + decent AMD motherboard (with PCIe x16 and IGP)+ DDR2 RAM will cost ~$120-150. Then upgrade graphics later. You will essentially be wasting that $75 as AGP is dead. Better save up money for a new build.
If you MUST get a GPU +1 for 4670.
Here is a cheap build (assuming you will re use HDD,case,etc):http://www.newegg.com/Product/Product.aspx?Item=N82E16813128342
Gigabyte 740G AM2+/AM3 (the curent IGP is better than that 5200)
AMD Athlon II X2 240
1GB DDR2 800http://www.newegg.com/Product/Product.aspx?Item=N82E16820211063
Total: $132. So for $62 more you get a more future proof PC than upgrading the AGP.