how far do you want to go? turbo upgrade options seem to be unlimited these days! certainly talk to Andrew at promax as he seems to get good reviews here (although i've never used him myself), and
www.vitesseracing.com seems to be getting awesome reviews over on rennist. Steer clear of guru themselves, as they don't seem to have very good customer service, but promax does their stuff, and you ca actually get hold of him!
other than that, just decide on your budget, and then add a shed load more for getting it fitted and tuned if you don't operate the spanners yourself!.
IMHO, in order of necessity for adding mods is:
boost enhancer (not needed if you go to boost control system)
3-bar FPR and chips (proper chips from guru, vitesse or lindsey. Not superchips!)
MAF
dual port wastegate & boost control system (either manual, i.e. a dial, or electronic, i.e. a flash HKS, greddy etc unit)
bigger turbo
after that, the sky's the limit!