There are lots of options for turbos. Ranging in price from $2500 to over $7000+. Of course you get what you pay for. A basic, basic turbo setup can ter you maybe 40 or so extra hp. Still pretty damn good. The sky is basically the limit. Take a look around, we don't have a lot of turbo info on here now as we just moved in. Check out some of the turbo cars at www.se-r.net. You can do exactly the same things to the G20 that you can to those cars, since they have the same motor.
Ray will still admit to being a bit of a turbo monkey. He did put together a good little budget system though. The guys you want are Rob Cadel and Mike Kojima. BUT, don't waste their time with that question. They don't have a lot of time on their hands. Do some research, get educated and come back with some imformed questions. Go on www.se-r-list.org/archives and look around for some post by Cadel and Kojima about turbos. You'll learn a lot. But be patient there is a lot of info to sift through there. Take your time with it.
Jason is Right! There are several turbo kits available to the SR20 community! You can buy a kit or pice a kit together! All the information you need is easy to find on www.se-r.net , www.sr20deforum.com , www.g20.net , etc....
All you have to do is hit the search button or look through the list archives!