For GeForce cards, it is important to note whether it is GTX or GT (or GTS). In general, for similar series, GTX cards are more powerful than GTS cards, which are more powerful than GT cards.
On another note, the M trailing behind the number notes for "mobile," which are usually less powerful cards (but probably unavoidable for mobile devices).
The numbers are almost always confusing. They do this to compete with other brands and to push sales. Don't chase the numbers.
For example, in most real life scenarios, you probably would find GTX 460 more powerful than GT 645, and GT 645 more powerful than GT 750M. You really need to refer to benchmarks that convey your purpose to make a proper decision.
Again (I really can't stress this enough), don't chase the numbers!