Category 6

If you want, I found this PDF online which gives quite a bit of information regarding Category 6 cabling. The main difference (as far as I know) is that it can transmit information at 550 MHz, while Cat5e can only transmit at 350 MHz, allowing for higher rates of transfer with larger data and at a longer distance. I may be wrong on those numbers (I've never worked directly with Cat6, though I have done extensive work with 5e). If you're looking at building a network, I would recommend Cat5e. It'll handle gigabit connections without a problem, and there's no need to spend the extra money for Cat6 (unless you're in an enterprise environment, where different cabling rules apply).

AJ