what engine? how much boost? what size dp back?

im running a turbonetics t3/t4 60mm on my y8, it all depends on what kind of numbers you looking for and what rpm you generally drive arround,

for example a t25 ( stock eclipse turbo) will begin to spool arround 2000 rpm on a sohc but will reach full spool arround 4500 rpm which means that after 4500 rpms you will not gain any more hp

the 60 trim i have on my car starts to spool arround 3200 rpm but will make much more hp per lb of boost, and has a much better powerband for my application

rule of thumd is 12 to 15 hp per lb of boost