nope ethernet cables can support upto 1gbps ..you just need to make sure that they are properly crimped.its better if you have bought them premade as they are factory tested.
OK if it's not working at all it could very well be a crappy cable or some other problem, what problems are you having exactly just limited or no connection at all?
How many PC's are on the network? Your ISP? Your networking equipment?
ahh sorry if you mis-understood there isnt a problem i was just curious whether or not i was getting as much internet as i can, thanks for the information and sorry again for leading you up the wrong road ;p