Question about video card naming

[darkstar845]

How are the names of current video cards related to previous generation video cards in terms of performance?

For example:

gtx 550Ti comparable to gtx 460?
gtx 570 ---- gtx 480?
Radeon 5770 --- Radeon 4870?

Another question, does AMD have more Video cards per generation/series than Nvidia?

 

[kajabla]

There's no set rule. Your comparisons are roughly equivalent, but in each case the previous-generation cards are slightly better, though generally not as efficient. This is particularly true in the 480/570 case: the 480 is notoriously loud and power-hungry, and the 570 is famously efficient and decently quiet.
Generally, the new generation steps up in efficiency and noise (excluding the flagship cards, e.g. 590 and 6990) and, as we see here, moves up almost one name-step in performance.
AMD has slightly more cards per generation, at least recently (the flood of 8800 and 9800 versions used to give Nvidia more), but not by much.