Well you'd better tell us what your r33 already has, or is it like mine and stock as a rock?
5g's inho would be best spent looking for the biggest turbo you can get and sticking it on, al la BelGarion style
Seriously though:
Mags with decent set of tyres
pod with cai box
turbo back exhaust with high flow cat (or no cat)
EBC
SAFC
FMIC
Thats about 5g's right there, right?
Well that is my plan, going to carry it out when i get my money tree