The wastegate corresponds to the turbo. In plain ole' terms, the wastegate determine the boost. (actually you do) But its harder to explain....vacuum and other stuff plays a role too. The gate is closed until you hit the boost level you have set, the air tells the gate when it gets to that level then the gate opens a certain amount to keep the turbo going at the same speed so your boost level stays constant. etc.

Anyways about the wastegate to choose, I think 35mm 1-7 psi, 38mm is go to 7-15 psi, 40mm is 15+psi. Im pretty sure thats how it breaks down. I wouldnt worry about getting any bigger than 40mm anyways. I personally liked the Deltagate myself.... but TiAl definately is better quality everyone seems to think.

Check this out anyways:

http://www.turboneticsinc.com/wastegates.html