Your G31 mb will have a pci-e x 16 slot for a gfx card
Later model mb's have a pci-e x 16 [ 2.0 ] which basically is CAPABLE of transmitting twice as much data .
Fortunately only very top end gfx cards can use all the capacity of the pci-e x 16 slot on a mb like yours , so you can upgrade quite successfully to a much more powerful gfx card .
What you should get depends in part on your monitors resolution . Higher number of pixels on the screen means you need more gfx power to get things flowing nicely
If you have a 19 inch screen then look at
4830 , 4850 , 9800GTX+
For a 22 inch screen then
4870 , 4870 1 gig , GTX 260 .
Beyond that then you probably need to upgrade the mb too
Have a look here
http://www.guru3d.com/category/vga_charts/
for the kind of performance that you can expect
30 fps is very playable , more than 60 fps does nothing because an LCD screen cant display more than that anyway
Your power supply might be an issue and might need upgrading too . The one you have isnt known as a quality brand . Personally I would buy the gfx card and see if it runs . Any signs of trouble though and I'd be off to newegg for a better PSU