Ah now, this is a question! Many people will tell you that because an HDMI signal is digital, you don't need to spend money on a cable and a £10 is just as good as an £80 one as a £500 one etc. etc. (due to the nature of the encoding and all sort of other things they point to). Personally, I believe in what I can see and I swapped my bundled HDMI cable for a Chord HDMI 1.3 Silver Plus and was very happy with the improvements over my previous cable (it's currently an Award winner in WHF).
Really, the only thing I can do is advise you to try a more expensive cable and a cheaper cable, and see if you can see any difference. So best bet is go to a local dealer and see If you can either borrow or get a no quibbles money back guarantee on both. That way, if you did see a noticeable difference on the more expensive cable, you can keep it and be happy. If you can't, you can take it back and get your money back. Either way you win.
Current recommendations I can think of for quality more expensive cables are the Chord as mentioned above, or the QED Qunex HDMI-P (which is a bit cheaper than the Chord). There are plenty of others though - have a look at the Buyers Guide at the back of WHF magazine.
Now, I await a lot of people to come in and tell you it's not worth the money!