You've made the mistake of thinking higher numbers equals higher performance. The first number, 6 or 7 in this case, indicates the family. For example, GeForce 7 cards are the new family of cards, to replace the old set. However, not everyone will be able to buy the top end expensive cards, so they need to release some slower, cheaper cards too, right?
Well after they have designed and set up factories for GeForce 7 cards, there is little point in spending money on seperate factories for GeForce 6 cards which use a different architecture and therefore need a different way of production. So they simply designed fast and slow verions of the GeForce 7 cards and mass produce them.
The 7300 is designed as the low end spectrum of the series for budget gamers. It is NOT faster than a 6800. The 7600GT is about equivilent to the 6800GT. So sorry, but I'm afraid you've downgraded. A lot.
(Sorry to rub it in but you really have made a mess of it)