Depends on how far you want to go
Think you have a TD04 as standard and Andy Forrest can do a hybrid TD04 which is good for about 330bhp and has very good spool up and virtually no lag
Then you have the VF series turbo's as used on the STi's, i have a VF34 which should be good for 350bhp, they spool up slightly later (about 500rpm) unless you have some ported headers as well which make it spool up just like a TD04 and this turbo pulls all the way to the redline
Then you have the TD05 turbo's (16g, 18g, 20g,), not sure about the 16g but these are bigger turbo's and will have more lag and spool up later but you will get a bigger hit and i think the 18g or 20g is good for around 380-400bhp at least
So it all depends on what you want, a fast everyday car that is easy to live with then i would go for a hybrid TD04 or VF34/35 otherwise big power from the TD05 18g or 20g but it will be more laggy
Why not check out Andy Forrest's website http://www.andyforrestperformance.co.uk/4436.html [Y]