ZIF vs. PCI upgrades are based on what machine you have, not which you want to buy. The G3 used a ZIF based processor, so that is what you'll buy as an upgrade. I have a Sonnet 500MHz G4 upgrade in my G3 (originally 266MHz) tower and the difference is incredible. I do a lot of video work. Some video compression in Media Cleaner Pro that took my up to 5 or 6 days is now being done in 8 or 9 hours. The price of the upgrade was way less than the price for another computer, which is what I was considering because I wasn't able to check e-mail for days at a time.
For day to day stuff, it's not so big of a difference. Given that you aren't big on video, etc., it may not be worth it. Down the road, as upgrades get cheaper and faster, it may make the difference more worth your while.