Good that it installed properly. When the iMac is lying face down on the display, you should install RAM in the top two slots first (the default factory slots), then the bottom two. Each pair should match:
Back of iMac
2 GB --- 2 GB
4 GB --- 4 GB
Display glass of iMac
If you heard a series of three beeps with a five second pause, that signals a RAM issue, usually failed RAM. If you hear one beep repeating every five secs, then it doesn't recognize any RAM.
You can install all of the RAM since OS X is successfully installed. Leave the two Apple sticks in the top two slots, and instal the Ramjet in the bottom two. Startup the iMac, and if things are normal, you are all set. If you hear any tones, or if it doesn't startup normally, then there is probably a bad stick or two. You can check in > About This Mac > Memory to see if all the RAM is recognized, and verify the two sets are the same sizes.
Hope that helps,
C