I have a 3.4L GTP, and i was thinking of addining a cherrybomb glasspack into the straight pipe under my car before the mufflers....good idea or no?

There is no point to doing that.

will it sound like s#$t or just wont do anything?

If your mufflers are still good, it won't change a thing, other than possibly making it a little quieter.

What if i replace the mufflers w/ 2 of them???

It will be much louder than stock, but I've never heard a car with that set-up, so I don't know if it would be "Ricer" obnoxious or not.