There wasn't a huge difference in GX to RX anyway. LX was the pov-pack model - it didn't have airbags, or power windows. The other models had dual airbags. Going to a place like redbook you may be able to see the features and compare. Alloy wheels, ABS, front sports seats, body kit, badge and a few items of interior trim were different between the GX and RX. I don't have any brochures that old so can't help you there. To my knowledge there weren't many "options" available anyway.
http://www.redbookasiapacific.com/au/ve ... =PSIMPREZA
Were you after a sedan or hatch and was it pre-August 99 or later?
According to redbook pre-August 99 Impreza hatch specs (A/C was option on all models):
LX Standard Equipment: 2 Speaker Stereo, Adjustable Steering Col. - Tilt only, Body Colour - Bumpers, Engine Immobiliser, Power Steering, Radio Cassette, Remote Fuel Lid Release, Tacho
GX Impreza standard features: 4 Speaker Stereo, Adjustable Steering Col. - Tilt only, Airbags - Driver & Passenger (Dual), Body Colour - Bumpers, Central Locking, Cruise Control, Engine Immobiliser, Power Door Mirrors, Power Steering, Power Windows - Front & Rear, Radio Cassette, Remote Fuel Lid Release, Roof Racks, Tacho
RX standard features: 15" Alloy Wheels, 4 Speaker Stereo, ABS (Antilock Brakes), Adjustable Steering Col. - Tilt only, Airbags - Driver & Passenger (Dual), Body Colour - Bumpers, Body Kit - F&R Spoilers, Skirts, Rear Apron, Central Locking, Cruise Control, Engine Immobiliser, Fog Lamps - Front, Power Door Mirrors, Power Steering, Power Windows - Front & Rear, Radio Cassette, Remote Fuel Lid Release, Roof Racks, Sports Seats - 1st Row (Front), Tacho