A video card with more memory isn't always better. A 512MB X1650PRO is far worse off than a 256MB 8800GT.
Look for the number of shader processors, memory interface, GPU clocks and the type of memory used. All these help decide whether or not one card is faster than another.
As far as I know nvidia never did that, ATi did. Nvidia had stuff like GF2, GF2MX, GF2MX200/400 (and others), GF3 (several versions), GF4 (several versions), then they went to the thousands in what would be the GF5, doing FX5xxx, then they dropped the FX for the 6000 series and above - that continues until the new stuff like the 280.In the past Nvidia has always labled its cards in said series x100-x800 and an occational x900 card. But that has changed with their new 200 series cards.
Lets take for instance the Geforce 7 series. which had a product line through 7200-7900 (7900 being the fastest of the lot).
Without doing research, it is hard to say if a 7500GT is faster than a low end Gefore 8 series card.
The gain in the series number is usually a big jump in capabilities. But you can have more capabilities and still be slower at games.