12Kb/sec is only 2x faster than dial-up, something is seriously borked or someone in the house is torrenting. If they're torrenting ask whoever is doing it to limit dl/ul rates on their client.
Is it that slow for everyone or just over wireless?
Sorry I'm no help with regards to external antennas the only ones I used were for commercial applications.
Might also want to try the parabolic thing suggested (example) at the expense of looking wierd.
http://www.amazon.com/2-4Ghz-Square-.../dp/B001934U4Y