Not a good idea man, I can't tell you exactly what your machine will do (some don't boot, some boot but OS doesn't start, in some, the RAM will just fall back to 667,etc) but overall its not suggested. Plus, for the gain of only 512mb, I seriously don't think its a good bet.
Get a 1GB module of DDR2-800 from an online dealer, these chips aren't that expensive and you'll get a lot more out of your iMac than with the 1Gb. I have the 2.0Ghz Alu iMac, and I keep track my RAM usage using Activity Monitor. The 1GB it came with leaves it severely under powered, and using HDD space instead, decreasing your overall perfomance. I put in another 1GB module I had lying around, so now I have 2GB, and the machine regulary uses about 1.2-1.5GB of that, so I'm perfectly sorted.