Here is the biggest bang for the buck upgrade:

Cost: ~$4.00

Change the value of the cap and re-wire the tone pots to:
Bottom: bridge pup
Middle: middle + neck pup

If you have a few extra bucks, you could get two different value of caps to try which one you like better.

For cheap, I prefer 0.047 Sprague Orange Drop.
For more $$, I prefer 0.047 Jensen PIO cap (~$15).

Either way, I don't think you can find a bigger bang for the dollar.

Have fun.