Ok, here we have 2 cheap items:
What I'm wondering is if the video card will make a significant difference over the onboard graphics. I think it would, but I'm just wondering whether or not it would be substantial.
The 7300 GT is a fairly weak card, but still better than onboard without a doubt. For the price it really can't be considered a waste and would make a good temporary card till you can get something better. Honestly though the 7300 can play some decent games like HL2, probably at medium settings at a resolution of 1024 x 768 at 30 fps. Newwer games like COD4 might even run well enough at low settings with the resolution set to 800 x 600, but games like Call of Jaurez or Crysis probably won't stand a chance. lol
For non-gaming purposes the card will be completely adequate and should even be able to handle streaming/playing 720p video.
Good answer Recovering. I have seen people say that onboard is fine if you don't play games and that is true from a graphics point of view, but onboard still chews up CPU cycles. A lot of them at times! An inexpensive card like the one he listed is a good idea for a non gamer. I even have a friend that plays BF2 and HL2 (As you mentioned) on his 7300GT.
Save the money and get a 780G board or wait for the upcoming 790GX; both are superior to your listed board and video card combined.