Save up your money unless an exhaust is completely necessary. I've come to realize that without patience, modding your car is not a good idea. You'll end up making the wrong decision because you are in a hurry. Take time to pay off your car, save up some money to pay for a full cat back exhaust.
Some decent exhausts for the G2's are:
-Greddy MX
-RS*R (very rare, probably wont find this one)
-DC Sports cat back
-Thermal classic
-HKS Hiper (a bit loud)
If you can't afford a full cat back, like the ones above (probably from 4-500 dollars), you could get a muffler and then have it re piped at a muffler shop.
I'd recommend waiting. Take your time and don't be in such a hurry. Pay off your car first.