1. If ur budget permits then out of 7300gt and 7600gt, get urself 7600gt without doubt....no need of comparision.
2. XFX is a good brand with one of the best service. As for a card being slow or fast, it depends on diffferent factors......like core & memory frequency, memory type, at what setting u want to play a particular game etc.
A company release different version of same product, so I would suggest u read the specs. before buying. As for other options u can get LEADTEK, PALIT, FORSA, ASUS and many more, only high-end card are a little difficult to find.
3. You just can't rate the cards u have listed as they just are not of same category. If u have the budget get 7900GS if not then I would suggest that at minimum get urself 7600GS/GT.
What's your budget?