Question should really be "I want to spend......what can I get?"
To start getting a few more BHP the first thing most modders do is to get a good exhaust/filter system in there, then drill the air box. This will probably gain you upto 10BHP, some decent plugs, leads, a good service and some quality oil are all essential when venturing down the power plus road too.
I have noticed that after switching from 17" wheels to 14" that the acceleration has improved, however this was not the intention, just a nice little bonus, however top speed is well down now too.
The next big step for some extra ponies for my cab is to go for a full engine swap with one of TSR's sports engines, but then this is "take my wallet and leave me starving on the street" territory.
However when all is said and done a Cab is for posing not racing.