I'm going to go against the grain and recommend CAT6. While theoretically CAT5 should work, it is not guaranteed and I have seen problems with high bandwidth use. Gigabyte ethernet requires that individual copper pairs work at 200mhz and should support 250mhz. It uses all 4 pairs in a network cable. A 1000mbps signal can theoretically be multiplexed over 2 200mhz cables. CAT5E cables typically work up to 350mhz which is more than enough and has more twisting. However, CAT6 cable provides plastic down the center of the cable to keep the pairs apart and so far has demonstrated a reduction in packet errors. It is slightly wider too though.

Having said that, I like the CAT6 cable for the obvious reason. If used, you wouldn't have to be as concerned down the road. CAT5E would be my next choice and would be more than you would probably ever need. I am not sure that you will really see any speed close to the cable's potential. There are just too many factors involved. Even a server with 2 1000base cards probably won't exceed speeds over 140mbps-160mbps because the speed of the media comes into play also. You can test the speed of the signal with a tester but in reality you will never see anything close to it because the equipment all along the way would have to be compatible and probably limited most by disk speed.

I have seen some Intel gigabyte cards actually support faster speeds but then remember the motherboard must support it as well as the read/write of the hard disk must be taken into consideration.

Whichever you go with, you can purchase a cable tester relatively inexpensively (make sure it is capapable of testing the higher speeds) and is well worth the cost. Even when we hire outside installers and they test and certify the speeds, we do the same afterwards and have never regretted it.