Is purchasing a 512MB memory card for my iBook G3 worth it?

I just purchased an iBook G3, 500/256/15/CD/OS X.4.11. It was a great deal at $100, in my opinion. I saw a 512MB PC133 SO-DIMM at OWC for $29.99. Should I purchase this upgrade, bringing my total memory to 640MB? (I would have to remove 128MB for the new one).

Also, I have a Power Mac G4 Dual 533/512/160/SD/OS X.4.11. The configuration is as follows:

SLOT 1 - 128
SLOT 2 - 128
SLOT 3 - 256
SLOT 4 - There is no slot 4.

All I can afford is to replace a single 128 with a 512. My total would then be 896. Is it worth it?

Thank You!

I appreciate your input, and I am certain that I will buy the memory now. Yes, 640MB is the maximum this specific machine (500/256/15 Late 2001) will hold. And 1.5GB is the maximum of the Power Mac (2x533/512/160 Digital Audio). Visit http://eshop.macsales.com to check out their low prices and quality upgrades.