I'll be swaping the engine on my 98 Hatch DX for a GSR engine and I was just wondering what's the most power you can extract of that powerplant while still maintaining it smog legal in Cali (emissions+visual inspection)? What products will allow you to do so?

The California Air Resources Board (CARB) has a current list of EO-qualified products at the CARB website at http://www.arb.ca.gov/msprog/aftermkt/devices/devices.htm

CARB sets the standard for all 50 states and evaluates hundreds of parts, testing them and passing those that do not add emissions.

Just an example of some CARB approved products:

AEM Cold Air Induction systems
A'PEXi VTEC AFC
Bosal headers for '94-'99 Integra
DC sports CAC intakes, TCS exhausts, and headers
Greddy exhaust and turbo systems
HKS street-legal turbo systems
Jackson Racing superchargers
JG/Edelbrock headers
MSD ignition kits
... and hundreds more

Check it out.