I don't know why N'Vidia changed the way they number their cards. Before we know x900 is the best, while x800 is great and the x600 or x500 weren't that bad.
Now, I have no idea what the numbers are for. Before I know 9800 is more recent than 8800. Now, it's all confusing.
Do I have to read on every single one to know which one is best for me? or is there a way to know what kind of power we're talking about by looking at the model? (for instance I wouldn't buy a 9200 for my friend but I would buy a 9600 for him after some research).
It's the same format but using GTX and three digit. The first digit describes the generation (2xx, 4xx). Second and third digit can be described as the tier (such as your x900 and x800 description). So a GTX 4xx card is more recent than a GTX 2xx card. A GTX x80 or x85 can be considered best single-GPU. Then it trickles down to other tiers such as x75, x70, x65, x60, etc. The only new placement is that "x9x" is saved for dual-GPU cards, such as the GTX 295.