Judging from the mainboard chipset i guess you're running some old core2duo and something like 8800GT (nvidia) or HD3870 (ATI) as your graphics card...
4GB RAM is plenty for an old PC like yours. You probably won't run much faster, because your CPU and GPU are slowing things down anyway. It's not worth to upgrade single elements of such a system anymore. Rather save up some more money and wait a few month more to get a new one.
