A 15 metre cable wouldn't have any slowdown effects at those sort of speeds.
As far as wireless goes it depends on what your walls are made of, if they're stud partition walls ( plasterboard nailed to a wooden frame ) then not very much, if they're 2 feet thick stone walls probably somewhat more of an effect!
As far as adaptors go, PCI cards are generally better in my experience than USB dongles both in terms of speed and reliability of connection. I don't know what the O2 wireless box is but any name brand ( Netgear, Belkin, etc., ) wireless cards should work. Probably better to go for the 54g ones. Other people may know of adaptors which work better or worse with the O2 box.
If people on here can't advise you of which cards to use then swing over to Thinkbroadband
and see if you get any joy over there.