If u're using Windows i'd suggest u to try cFosSpeed (www.cfosspeed.com), it does traffic shaping on windows and has l7 detection, i use it at home where among me and my housemates we have about 10 computers connected to the same line, and everything works fine, even if we have a highly asymmetric line (20Mb down/1Mb up), and even if we have several P2P programs working at the same time.
Other than by l7 detection it can prioritize traffic by the program's filename, which is very useful since many P2P programs today use data encryption, which makes l7 detection difficult. However, this is possible only on the computer the program is installed on, so best solution is to have one computer online 24/7 to share the connection, shape it with cFosSpeed, and also have any P2P programs installed there, so they'll get the proper priority. And by the way remember to raise the connections limit in XP also.
Only drawback? it's shareware, but not expensive indeed and u can test it for free for 30 days.