Sorry, but that's a socket 939 board so you don't have too many options for a CPU upgrade. Dual core chips for s939 boards cost a pretty penny, but if you just really want to keep the board you could hunt on ebay for a socket 939 4200X2 CPU or something. Of course, it will cost you more than a basic AM2+ motherboard with an Athlon II 240.
As for the GPU, anything you buy now can be used in your next build. If you want to keep this machine for a while though then there is no point in buying anything faster than a Radeon 4670 unless you upgrade the CPU.
The 512MB version of the 4670 has slightly faster RAM and is thus a little faster in most games and settings. Still, for the longevity of your system it may be best to go with the 1GB version. Anyway yes it would be compatible with your motherboard. It is fairly straight forward to install. First download drive cleaner pro because older Radeon drivers can leave alot of stuff behind. Uninstall your drivers for your current Radeon. Reboot the machine into safe mode and run drive cleaner pro. Turn off your machine and replace your old Radeon with the new one. Turn on the machine. Install the drivers for the 4670. Be happy.