I noticed the other day that my muffler its got a leak...
What kind of problems can this cause?...if any
should I park it till i get a new muffler

Its just gonna be a bit noisier, and will be a place for additional corrosion to start. It may also let exhaust gas escape under the car, and could possible enter the passenger compartment, but being far enough back on a thirdgen this should be much a problem, if any. It won't hurt anything, I would take the oppertunity to replace with a performance piece.

just a bit noiser. wont sound as good as properly working muffler also... it will sound like a leak and not the muffler tone. replace it

Ready for it? It will sound.... better.
Just that's my opinion, but on certain cars (V8 third gens) they sound better with a few rust holes in the muffler.
so yes, louder. Exhaust leaks will be within 10" of the regular dump, so it's *highly unlikely* the fumes will do any *more* damage to humanity (namely - you).

It doesnt eat/ lose any HP though or gas milage?

nope