Option 1:

Get a new graphics card, it sounds like an expensive prospect but it's honestly the best way to improve your video performance.  There is software out there that will let you tweak and overclock your current card, but improvements are going to be minimal if you card is allready overtaxed.  Check out sites like newegg.com for their monthly specials, finding a good card at a discount price is easy if you check the site every week or two.

Option 2:

Adjust your game settings, it's amazing how changing just a few options can make a game playable.  First off lose the shadows, of all the graphics options this is the least useful and will quickly improve your fps dramatically.  Viewing distance is important if you are playing a fps but it's a secondary concern when playing RPGs.  Resolution, most games will auto set to the best resolution based on your hardware specs, but you might have to sacrifice screensize for playability.  And lastly Texture Quality, this is the last option I would choose to change because it really changes the look and the feel of the game, but you gotta do what you gotta do.

There are  multitude of ways to enhance what your current video card's performace, but to be perfectly honest it's all about hardware.  Get a better video card, more RAM and a faster CPU.  If you are smart about your upgrades you can usually get away with upgrading one or two components at a time.