Q: This goes to how easy Apple makes their computers to upgrade RAM CHIPS! Now, one major question: Is it a better choice to buy a used Intel-processor iMac for $400-600 or spend the $100 for a 2-GB RAM upgrade?  :

Depends...  If the G5 iMac is still working well and doing everything you need, then spending $100 to more than double its RAM is probably worthwhile.  If you want access to a later Mac OS X release and associated Intel-only software, then upgrading to more recent Intel hardware makes more sense.

NOTE:  Just because it's Intel, it does not mean it will run the most recent versions of Mac OS X.  For example, the first Intel Macs with "Core Duo" (not "Core 2 Duo") processors cannot run Lion (10.7).  The upcoming Moutain Lion (10.8) drops support for more Intel Macs.  This web page has the official list of supported Mac models for Mountain Lion

http://www.apple.com/osx/how-to-upgrade/

(Step 1)

What do you do with the machine?   Developers are dropping support for PPC machines. Do you need to keep up with the lastest software?

I do not know your exact iMac model, but you may be able to get the memory for less ... $50.

http://eshop.macsales.com/search/PowerPC+G5+iMac

Give them a call.

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Could you describe your intel option some more?

are you buying a used machine?  logic board -- I've not heard of this possiblity?

Robert