The quickest way to bump up the 4.6 would be to swap in Proformance Improved (PI) heads, cams and intake... that'll bring it to whatever the PI 4.6 is at.. I think 260hp/310tq? People have said it's around $1k... then headers, underdrive pullies, larger throttle body etc...
Basically any mods for the Mustang SOHC, will go on the Town Car.... and likewaise, any suspension/handling/chassis mods for a Crown Vic will fit the Town Car.
Here is a good example of what some reasonable mods will do... there are people that put turbo or superchargers on them too...
http://www.cardomain.com/ride/274907