Blacken, this isn't particularly directed at you, so sorry for the reply.
To all people who have posted in this thread: READ THE ORIGINAL POST!
To answer your question, the 7300GT that you already own will ravage, savage, pillage, pwn, or whatever word you want to use, your onboard graphics. You will see a big difference in performance if you crank up your graphics quality just a little in the games you're playing.
That being said, that card is not a powerful graphics card. WAY better than any onboard solution, but that's really not saying much. I have an 11-year-old Matrox MGA (yes, I am that old) that would just about keep up with onboard graphics, if it only had 3d capabilities. But I digress.
There is no reason to go buy a more powerful card if the games you listed are all you ever play. The 7300GT will be fine. There's really nothing around that same price that'll beat it. $50 more and you could get something really good (like an X1650 or 7600GT, as suggested earlier). Neither would really be required to play what you want to, though.
I hope that this helped some.
-J