Yes, mount some fresh tires ASAP because 12 yr old tires suck! Ideal sizes for those wheels on a 986 are 235/40/18 and 265/35/18.
Pretty fun discussion on tire diameter differences though. "Twitchiness"??
All 987, 997, 981, 991 run 1" smaller diameter in front than the rear. It adds a little rake to the car so there is more weight on the front tires. This helps to generate excellent car balance in the corners. The idea that all these cars are "twitchy" because of the different diameter has never been discovered until now.
If your car feels twitchy, it needs an alignment.