I had Comp XE270AH cams in my '03 GT. I don't know what the exact gain was, but with just cams, CAI, headers, plenum, TB, and pullies I put down 290rwhp/301tq. With the cams, you should install good valvesprings from Comp Cams. If you are going with a blower down the road, try the XE270AHs from Comp or the MPH cams. If you are staying N/A, I would highly suggest the VT Stage II cams. There have been PTV issues with the Comps and quite a few guys on Modular Depot swapped their Comps for VTs for that reason amongst others.
As for gears, you don't have to have them, but my car didn't really have enough gear with the 3.73s. It would have loved a set of 4.10 or 4.30s to keep the car in the sweet part of the power band at the track. Even at that, I still got a respectable 12.90@105 with a 1.89 60ft. The car still had more in it, but to have gotten the full potential out of it, another gear change was needed.