The rule higher the number the better works when you're comparing within the same generation of cards, ie. 9800GT > 9600GT. The generations pretty much go in order, 5500, 6600, 7600, 8600, 9600, and now the 200 series.
Yeah.. The most expensive consumer level cards are the GTX 295 and 4870x2 which are nowhere near that expensive, you're thinking enterprise cards for CAD work and the like..
GTX200 series GTX295>GTX285>GTX280>GTX275>GTX260core216>GTX260core192(all better then 9000 series)
GTS200 Series GTS250(Re branded 9800GTX+)>GTS220(between 9500GT and 9600)
9000 series: 9800GTX+ 9800GT>9600>9500>9400
I can't stand the whole numbering scheme myself. It seems all a plan to make you think that the more numbers the better and if its got an X then great. I wish they would go to a number plan like 1, 1A, 1B, 2, 2A, etc. but they probably wont because the way they have it now is good for confusing people into paying more.