It all depends on what sort of computing you are doing. Gaming would require more RAM, however if you use your machine for typical home media applications (watching movies and streaming music via Network Attached Storage NAS, light gaming like polar bowling, document editing, and browsing the Internet) then 4GB will get you pretty far.
If you still want to upgrade your memory then a new system board is one way to go. If this upgrade is based on increasing speed then there are other inexpensive ways to upgrade. Ready Boost is an interesting feature in windows Vista and 7. You can insert a 4 or 8GB flash drive in an available USB port and tell windows to use it for Ready Boost. It basically uses your flash drive as RAM. You can also clean up your system by editing the registry, deleting temp files and uninstalling rarely used programs.
If you still just flat out want a new system board. I would recommend Intel's Media
Desktop Boards. However if you want to reuse the processor on you current board try ASUS' M4A785 System Board
or Biostar's A780L System Board
The one from Asus supports up to 16GB of RAM (with a 64bit OS) but is more expensive and the board from Biostar is less expensive, but supports up to 8GB of RAM. That's one other tid bit I'd like to mention. If your running or plan on running a system with 4GB or more of RAM, it must be a 64bit Operating System. Windows Vista and 7 32bit run okay with it but lack in performance in some areas.
Hope this helps you some.