A chip can increase your performance... but not very drastically. If you are looking for a drastic change I would go with a Supercharger or NOS, but that can mess up your engine. If you have your mind set on a Chip, I would check out Saleen's POWERFLASH Automotive Performance Computer. It isn't exactely a chip, but its better. I don't know if the POWERFLASH works for the '98 GTs but if it does it should increase your HP by 20 and also increase the driveability. You should be able to find some info on the POWERFLASH at www.saleen.com.
If you want a chip I recommend getting on yahoo and doing a search for something like Mustang chips.