Q: ddr2-pc4200 800Mhz on my iMac G5 late 2005

Usually if you have to different speed (MHz) memory chips, the computer

will see them as the lower of the two. Or, a faster chip will only run as quick

as the bus was designed for.

Also, be sure the pin count is correct; there are other spec DDR2-PC4200

with a number of pins that will not match the RAM slot count; & won't work.

{One spec sheet from a second-rate memory chip supplier, for portable RAM

suggested the 800MHz chip was backward compatible (so long as pin count

and memory shape could fit, plus other specs required for the chip to work)

in much slower memory applications. But it would not run any faster than the

slow bus data rate. -- Also, be aware of a consistent suggestion that one

should upgrade RAM memory chips in matching sets, re: speed.}

The iMac G5 12.1 you have may also go by the identifier 'iSight' as that may

not be a reference to a Late 2005; the model prior to this was 'ambient light

sensor' iMac G5 8.2, -- & first iMac G5, model identifier 8.1. (mactracker.ca)

As inexpensive as these chips are, I'd try for matching; known good pulls or

new, from reputable mac-centric sources.

Good luck !