Most likely not - you need to look up your model identifier in both machines and then look up the specs for the RAM for both to make sure. This is a good place to look up your model identifier:
(how to find model ID)
Aside from that, the new iMac has 4 slots; it should have 2 x 2 GB matched horizontally and you should add a matched kit in the other two slots. I have 12 GB total (added 2 x 4 GB) which works well with Lion and Mountain Lion (both a memory hogs).
No, you can't use memory from your older iMac, as your early 2008 iMac used DDR2 memory and your 2011 iMac uses DDR3 memory.
I installed another 4GB (2 x 2GB) in my mid-2011 iMac, and can see where the system uses >4GB. For what I run, 8GB works fine. I bought Samsung memory from OWC to match what was shipped with my system. A 2GB Samsung memory card is $17, and Crucial is $15.