Can I add memory to my iMac?

Maybe but without any information about your iMac all I can really say is read your manual, everything you need is in it.

Go to OWC page for iMac memory. Click on your iMac model and OWC will show the kind and combinations of memory you can add to your iMac. You don't have to buy from them of course but those pages will give you an summary (and OWC's prices) for what is possible for your machine.

OWC also has instructional videos on how you add the memory to various models of imac (among others).

This page will tell you everything you need to know:

http://support.apple.com/kb/HT1423

FYI...

Be aware that previous post references the various configurations by apple at the time they posted those specs. Technology moves on. So, say, where apple says the max ram is x, in more recent models it can be 2x. For example, at the time the specs are made only 4GB dimms may have been on the market and that is what apple tested with and published. Eventually 8GB dimms came on the market which work perfectly well in later imacs but apple never updates their pages or is late in updating them.

These days there are even 16GB dimms, not for imacs I don't think, but the technology keeps advancing.