7300GS is a weak, bottom-end card. Gaming value is very limited.
7300GT is built around a crippled version of the G71 core also used in the 7600GS and 7600GT, and is strong enough for light gaming at lower resolutions.
The 7300GS and 7300GT are very different cards because they have different cores. I feel NVidia should have used a different number for the 7300GT, such as 7500.
There are models of the 7300GT available in Asia and Europe with high clocks and DDR3 memory, and they are great performers for the price. Leadtek lists such a model for North America, but it could be very hard to find.
If you live in North America, your best bet could be to watch for a heavily discounted 7600GS, as this card (as well as the 7600GT) seem to be quietly discontinued in preparation for the upcoming release of the 7650GS.
If you want to make up your own mind:
7300GS
7300GT
7600GS