Yes, yes you can.
Will it work? well... that depends.
I can throw an old cyrix chip inside my computer case for some added transistor count... pretty sure it doesn't do anything but rattle around in there as there is no place to plug a cyrex chip in.
On a more serious note... if you have a modern mac then it has an Intel proc. If you want to upgrade, be aware that it will be the end of your warranty, and you will fall from the grace of Jobs (and we all need jobs right now). You will also find that it is damnedably difficult to work on most macs, especially those silly laptops. If you have a cheese grater case then you can add a 2nd proc, but you will also need the custom apple-only cooler that matches your particular case that you will never find in a million years. Then there is the whole business of the platform being aware of it's settings, and potentially locking you out of a modified system.
So on second thought... no, no you cannot. And dont get me started on memory in macs, they are super picky. About the only mod you can make safely is swapping your HDD... oops, Lion is a web distro only, so I guess if you want to upgrade you system drive then you are out of luck again! Besides, it is too hard to get to the HDD anyways on most of them.