Do you have a stereo sound setup (ie not surround)? For music producing you should do, and providing that is the case the M-Audio 2496 card will suit you better (about £50). Its a stereo card only and your money goes into quality rather than special features and surround capability. It is geared to recording work.
In any case plugging the guitar directly into the line-in is unsatisfactory, the guitar output level is too low and will not provide a suitable impedance match, the result is excessive noise. A simple but still poor way to do it is to connect the guitar to your amp then join the amp's headphone socket to line in.
Good ways to do it are to use either an FX board such as Boss ME50B and connect its master outs to the line in, or use a unit such as the Behringer V-AMP or Line 6 Guitar Port (designed for this application). Some units combine the soundcard function and guitar pre-amp/FX into one USB device, but will offer less flexability in the long run (ie. Zoom G2.1U)
