The second number in the series is the most important for raw performance. A 750 will be more powerful than a 650, but only slightly. The "7" means its a newer series, with improvements, but the series is a poor way to judge RAW performance. A 650 for example will be faster than a 730. So, you have to look at the second number for relative power or performance within that series of card, keeping in mind that the higher series numbers mean a more modern card.
The best, and only fool proof way to know a cards real potential is to check benchmarks of the card running the games you plan to play on sites like this.