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. On this right moment im connected with a CAT5(E) and Gigabit connection works fine, but i loose 6mb/s comparing with CAT6 cable.
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