There are a number of mods that do this already; I recommend using those.
The XP cap is there partly to enforce the balance of encounters in BG:EE, and partly to make sure that when you get to BGII:EE you aren't too powerful for the challenges there.
I used this. Very simple to use, no actual installation required (just a few override files), and I could set the new cap to whatever I want. Played through several games with the cap raised to 250k and encountered no issues.
Comments
The XP cap is there partly to enforce the balance of encounters in BG:EE, and partly to make sure that when you get to BGII:EE you aren't too powerful for the challenges there.
http://forum.baldursgate.com/discussion/10212/a-simple-xp-cap-remover/p1