Or just read this: Transmission FAQ: Read if you are thinking of upgrading!
And this section applys to 02-07s that are thinking of swapping in a STI 6 speed.
What's involved with swapping from a 5MT to a 6MT?
You will need to obtain and install the following components:
1. A complete 6MT transmission.
2. 6MT transmission mount.
3. 6MT shift knob, 6MT shifter linkage, and 6MT shift boot.
4. The drive shaft from a 6MT or the drive shaft from any model automatic transmission equipped Subaru. The stock WRX 5MT drive shaft may also be modified (will need to be shortened) to fit as well.
5. New rear end options:
a. New ring and pinion gears to upgrade the stock WRX R160 rear differential to 3.90.
b. Convert the 6MT center differential from 1.0:1 to 1.1:1 and use the stock WRX R160 rear differential.
c. Swap out the stock WRX R160 rear differential to a stock STi R180 rear differential.
The real decision to a 6MT swap is the rear differential, as going with the R180, though the "better" choice, raises the costs significantly as you need to use the entire rear end consisting of the STi driveshaft, STi rear half shafts, Sti rear hubs, and STi rear discs to include the Brembo brakes. This will also require the use of 17" wheels as well to accomidate the bigger brakes, which for some, may be an additional expense. As well, www.gearboxtech.com
sells the parts that let you use R180 inner CV joints and the R160 outer CV joints if you want to use the R180 rear end and not swap over to the R180 hubs/brakes.