Microwave antenna are pretty picky things to get to work. You cannot just use wire you must use microwave rated coax or you will lose more signal that you gain. Even with microwave coax you lose a large amount of your signal when you use the thin stuff that would be needed to connect inside the router.
I would buy a antenna that is designed to hook to the internal boards that are put in a motherboard. They are the same cards used in laptops. You can get a 9db antenna and a short connecting piece of microwave coax for under $10. They are not deisgned to solder but I guess you could. The solder connection though must be perfect to not cause signal loss at the microwave frequencies.
Best bet is going to be to replace the router. 802.11n does not have more range but it will transfer more data at a given signal level. So your signal level will not increase but your data rete will. You could also get a router with external antenna and you could try to upgrade them...it may help a little bit.
Your problem though may not be signal strength it may be interference. Try changing the router channels and see if you get any improvement.
Thank you so much for the answer! I bought a 300 mbps dual antena router. The signal is not increased much, but I'm getting much better speed per unit signal signal strength. I think I'll add a parabolic reflector to the antennae, as well. Thanks for your help.