Just bought Casio PX330. Great keyboard but not too crazy about the sustain effect. Would buying a better pedal give me a better sustain effect or is that function of the keyboard itself? Thanks.

Half-pedal (and quarter-pedal / three quarter-pedal) are features of both the pedal and the keyboard. If your keyboards supports this, but your current pedal doesn't, a different pedal might help.

I can't think of any other feature where the pedal (instead of the keyboard) makes a difference. I believe a "sympathetic resonance" simulation can make a big difference in how realistic the pedaling sounds but this is entirely a feature of the keyboard: if your keyboard doesn't have it, a different pedal is not going to help. If your keyboards does have it, there might be a setting to decrease / increase the effect.

Damper noise and pedal noise simulations might also help a bit. If you keyboard has this, there should be settings to enable / disable it somewhere.

I had a PX-320 ... changing pedal made no difference
in quality/quantity of sustain.

Thank you to both of you.

The sustain is done in the keyboard. The foot pedal is just a switch.

(I'm not talking about the three-pedal majigger that comes with the fancy stand for the Casio. I'm talking about the normal footpedal that comes with the keyboard.)

The PX-330 supports half-pedal operation with the SP32 pedal, and there is some user adjustment possible.