On my 98 HX, noticed that sometimes I'll get a check engine light because of the pressure in the gas tank, is it a good idea to purchase a vented gas cap or stick with the standard one?

 

The cap has to be sealed. If the pressure in the tank stays at zero (a vented cap, or no cap), a code will definitely be thrown.

The standard cap may have a pressure / vacuum relief valve to prevent damage to the tank in unusual situations. It should nor vent under normal conditions.

 

So it's probably in my best interest to get the OEM cap? Or will any similar cap be good enough?

 

OEM cap is best, and they are not expensive at all. If that doesn't fix it, there is also a 2-way valve on the side of the fuel tank (lets pressure out to the EVAP, and/or lets air into the tank as the fuel level drops. Beyond that, you are looking at various seals between parts of the fuel filler, fuel tank, and evap canister.