What TheQuietThings failed to tell us was that the Grand Am GT he raced was built in 1992, had a slipping transmission, and when he said it had a chip, he meant it had a Frito Lays chip clogging the air filter.
The Scion tC is definitely a move down in class from the Grand Am GT. With a 5 speed and a good driver hammering through the gears, it can do 0-60 in around 7.8 seconds, which is very close to the Grand Am GT's 7.7 seconds...though neither of these cars are hardly fast when compared to, say, a GTO or Neon SRT-4. There's no way you're going to see even an 8 second 0-60 time with the Scion tC and an automatic. Plus, once you start hauling other people around with you in your car, you're really going to miss that extra torque.
Of course, there's more to a car than its raw 0-60 time. There's handling, and the Scion isn't going to outhandle a Grand Am GT. The stopping distance of the Grand Am GT is shorter than that of the Scion tC. Then there's features, which the Grand Am GT comes with tons of...including that excellent 8 speaker Monsoon stereo system.
Bear in mind that the Scion tC is meant to compete with cars like the Cavalier, Sunfire, and Cobalt...the compact 4 banger cars. The Grand Am GT was designed to compete with the likes of the V6 Honda Accord and even some BMW models.
Finally, consider how you're going to drive it. If illegal street racing is your only goal, as I mentioned, there are faster cars out there. If, like me, your daily driving is in stop-and-go traffic with weekend trips down twisty, hilly, and sometimes bumpy roads with friends or family in tow, you will want the combination of a V6 with a 4 speed automatic. I'd stick with the Grand Am GT until you can afford something at least in the same class.