Q: IMAC not booting

Try an smc reset.

Boot from your installer DVD and run Disk Utility on your boot drive.

Also on the installer dvds you have the Apple Hardware Test.  You might want to run it to check your machine which includes that memory.

No backup of your boot disk either?

You are pretty handicapped here.  You can't boot.  No way to validate the hardware or the drives if you don't have the boot dvds or a external bootable backup.  Not much you can do at the moment other than getting replacement disks if that is possible.  Call apple to see if they can give you the replacements.

Alternatively maybe you could buy the retail snow leopard and try an update install.

Nothing I haven't mentioned.  If it doesn't boot and you can't do anything at the moment, it's basically a door stop.

Ok, one last desperation move.  Try booting in safe mode.

What the heck, throw in a pram reset for good luck.

Overlapped posts.  See above.

Turn power on and get a single bleep

More like a "beep" ???

That's RAM....

Power On Self-Test Beep Definition - Part 2

Replacing RAM is way better then a door stop.

BTW, just because RAM works in one Mac, doesn't guarantee it will work in another.

Replacing RAM is fairly inexpensive and something you can do yourself.

You can use Crucial's scanner > SDRAM, DDR2 and DDR RAM memory upgrades from Crucial.com

iMac: How to remove or install memory

Which begs the question from a statement in the original post.  Since you were fiddling with the ram, make sure it is properly seated.  Or start pulling sticks to see if there is a bad one that can be found.

As Carolyn says, the sound indicates a RAM problem: http://support.apple.com/kb/HT2538?viewlocale=en_US&locale=en_US

Have another go at reseating it. There was a thread on here recently where someone was convinced the RAM was properly inserted but proved not to be the case. He had one last try pushing really firmly with both thumbs at each end of the RAM stick, and he felt it go in just a fraction more. After that everything was fine.

I'm not sure if I do find the OS CD that the MAC will boot

Good point.

Well try removing some of the sticks and see if one of them is the problem.  Use one at a time.  May not be enough memory to do anything useful but see what affect that has on the boot.

If the dimm slots are like the ones I am familiar with there will be a tab on each end which pivits up when the dimm is fully seated in its slot.  So check those tabs.