Give a search thorugh the forums for Memory.
The CAS latency in general will not make a difference. There has only been one report of a difference in performance with lower latency, and the method of testing was flawed. Apple has never indicated tht their motherboards can take advantage of faster RAM timings.
The iMac is the most finicky recent Mac about memory compatibility
PC generic PC3200 RAM may or may not work -- unless the Serial Presence Detect settings are correct, it will probably fail in the iMac.
You are looking for a reputable seller who tests and guarantees compatibility with your specific model Mac, in writing, and offers a lifetime warranty and a no-cost return if it doesn't work (no restock fees). This would rule out most eBay sellers, big-box stores and online discounters.
MR readers variously have recommended Crucial, OWC and
Data Memory Systems as good sellers in the USA.
Thanks
Trevor
CanadaRAM.com