7300GT:
Core clock: 350MHz
Fragment pipelines: 8
Shader ops: 2.8 billion/second
Memory clock: 325MHz (650MHz effective)
Memory bus: 128-bit
Memory type: DDR2
Memory bandwidth: 10.4 GB/s
7600GT:
Core clock: 560MHz
Fragment pipelines: 12
Shader ops: ~6.7 billion/second
Memory clock: 700MHz (1400MHz effective)
Memory bus: 128-bit
Memory type: GDDR3
Memory bandwidth: 22.4 GB/s
^If any of that's wrong, someone correct me please. I'm trying to learn all I can about this stuff.
If you've got the money for the 7600GT, there's no question that it's so much better than the 7300GT. Definitely get it if you can.
And I've never heard that the XFX cards were really slow. Not to say they aren't, I've just never heard that; I personally have never used them, so I don't know. Generally speaking, brand doesn't matter, but sometimes companies will factory overclock their cards to a safe level, so if you don't feel comfortable doing it yourself, those will be a little faster. I like evga for nVidia cards, but like I said, it really doesn't matter too much.
EDIT: Also, a nice comparison guide in-game between the two can be found
here. For some reason, it auto-highlights the 7900GTX in blue, even when I selected the 7300GT. But whatever, you can still see the vast differences in game between the two. Just...don't go by the ones highlighted in blue.
And yeah, they have a lot of games at loads of detail settings, so it shouldn't be too hard to notice a wide gap.