GSR motor is an excellent motor, while not as potent as the b18c type r it is still excellent
as far as mods go, you can start with a good intake, header and exhuast system, then a good intake manifold and throttlebody, after that your ready for some cams and engine management.
here is my choice of parts and the average price you will find them for:
step 1
aem/fujita/injen intake $200
edelbrock/greddy/DC header $300-400
any 60-65mm exhuast you like $400
step 2
BBK intake manifold $290
AEBS 68mm throttle body $150
step 3
skunk2 stage II cams
skunk2 valve springs and retainers
your choice of engine management.
this order of upgrades has been proven many times on a gsr motor. usually i dont recomend intake manifold and throttle body so early but the GSR engine really benefits A LOT from those 2 mods.
with step 3 if you have the money you can even add some integra type r pistons that should bump your comression to 11.3:1