Literally any that is as fast as the IGP or faster. So *600, *800 or *900 series, where * can be 5, 6 or 7.
If you can set how much RAM the IGP can use in the BIOS, try it, it may help instantly.
Another good solution would be selling the extra 8GB of RAM as you will never use it anyway, unless you tend to do unusual things (like running 3 or more virtual machines at the same time, compressing more thatn 4 files using LZMA/Ultra (with 4 or more threads) simultaneously etc.).
CPU: AMD Phenom II X4 810 @ 3120MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 835/1135MHz | Display: L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 3.11 Pro x64