This is an age old question that has been addressed about a million times.. A quick search will give you a ton of discussions on this SAME TOPIC.. Please utalize the search button next time..
As to answer your question, you first have to give us some info as you can as expensive as 10,000 for a full PPG Dog with PPG Front LSD.. Or you can go the cheap route and get some RA spec gears for about 3,000 or so... The main factors that determine what you get are..
1. Your driving style
2. Your ultimate power goal
3. Events (if any) that you participate in.
4. Are you willing to sacrafice a little noise for power handling? (Dog Box)
5. Do you want a 5 speed or 6 speed?
Before any serious answers can come from this post you need to give us more info..
02 WRX.. "It's fun when it runs.. Problem is that it hardly ever does..."