450bhp is usually the area the w58's like to eat themselves so when you get there dont launch too often.
Turbo selection is usually done through your current power aims and future goals. If you want to upgrade later and don't mind replacing the turbo, best bet is to get a smallest turbo as possible to hit that bhp figure, therefore you have the quickest response at your power goal. BUT, turbo's aren't cheap, and if you only want to buy once and keep room for upgrading you will have to get a slightly bigger one, though the bigger the turbo the more lag you will get, so it's a toss up between power and spool.
Do some research about turbo sizes and AR sizes and also the best way to help you decide a turbo, is see what other people's set up's are like with different turbo's, what they talk about spool and lag etc.
Alot of people on ClubNA-T use the T61. I know craig on here (jekyl) used a garrett GT40 something (cant remember 3rd and 4th numbers) and said the power was good but had alot of lag and felt it was too big for that set up. He ran 378bhp on 7psi (i think it was same turbo but def stock internals & injectors). So remember its not just the pressure of air, but the volume, so the size of the turbo is important as well as power much PSI you push through it.
Turbo's Explained by Garrett
The above link has loads of good information and helped me understand what the AR rating means with turbo's. Have a read and keep your eye open on other NA-T members set ups and try and see what you come up with.
Sorry I went on a bit I get a bit carried away with NA-T stuff