The easiest thing to do is run MSI Afterburner from
http://event.msi.com/vga/afterburner/download.htm
After you install it, you can use the OSD to display the framerate, video memory utilization, and GPU utilization while running a game that you would normally play at the settings and resolution you would like to ideally run it at. If your framerate is not where you'd like it to be and the GPU utilization is at 99-100%, then you could definitely benefit from a faster graphics card.
You can repeat this process when you are up and running on your new graphics card and see if the CPU is bottle-necking it. Just do the same thing but also have the Windows Task Manager running in the background so that way it tracks your CPU utilization.
For example, I have a AMD Phenom II X4 at 3.534 GHz and a GTX 560 Ti OC to 985/1970/2155 and running any game at Ultra settings at 1680x1050 (my monitor's native res) does not fullly max out the CPU on any one core on any modern game except for Crysis 2. That game with the DX11 patch, Ultra Settings, and Hi-Res Textures will max out one core with the rest at about 85%. But still very playable framerates for sure. In my case I upgraded from a 9800 GT to this card (with same CPU) and the upgrade was well worth it.