RULE OF THUMB... WELL, FOR THE MOST PART, or at least, what I have been able to understand in a few years.
The lower the number of the Video Card the more economical or Bargain Bin it will be, the higher the better.
I also believe that this is true for better performance... GTX, GT, GS, GSO
As an example, you have the 7600GT, a good upgrade (worth spending money on) would have been a 512MB 8600GS or GT, 8800GS, GT or GTX and anything below that is a waste of money... but you now know that so we won't beat you up to much
Don't get discourage, try to move the card on ebay and buy a better one.
Now, you must be careful though, if you have an Iffy PSU, a bigger/better card may not be pushed by the PSU and you are doing even worse now.