Personally I have had the chance to test CAT5 and CAT6 cables and these lasts are better. I tested 5 different CAT5 cables and none could achieve CAT6 speeds, not to mention that 3 of them could not do even do 100MBIT. Cat6 cables have higher bandwidth, so they are recommended to avoid problems on gigabit connections.
We are not inside the CAT5 cables, so no one knows if it is a "good" or a "bad" cable, it's more safe buying CAT6, for sure you won't have any problems.
Now about the wireless nic N you talked, what model is? That's really weird, the best speeds that i could get was with Intel and Broadcom chips