installing 667MHz memory in a 533MHz Imac G5

Assuming everything else is matches (240 pin etc..), can I install a PC-5300 memory stick into a PC-4200 slot? Will it scale down? The older smaller 533MHz are harder to find and way more expensive. Thanks.

If the specs otherwise are the same and it is Apple compatible memory, yes Apple System profiler will simply recognise it as PC4200.

For example I use PC3200 in my FW800 and ASP recognises it as the stock standard PC2700. Here is a link to OWC memory and the 533MHz seems pretty cheap:-

Memory Upgrades for your Apple iMac G5 (2.1GHz 20-Inch iSight) at OtherWorldComputing.com

Probably shouldn't, I would imagine that a manufacturers spec is set for a reason, you will feel reassured to know that what you are using is working.