Customization....

So, i guess you can say that i am a new mac user, i have owned an iMac since may so not to long ago. I love my mac, a lot better then my old PC, although i find one thing frustrating. Coming from a old PC user, i loved customizing my PC. With my mac, it is hard to fully customize it. From my knowledge, customization on your mac is very limited. So far i have only installed an extra gig of ram (2gb). I am fairly new to the mac world and i was just wondering what customization options us mac users have. Can we change our video card, add more ram, etc. etc. etc. etc.

Also i was wondering on what to upgrade just to keep up with things. To clear that up i mean what's the recommended ram for my mac, what's the recommended HD memory and etc. etc. etc. etc.

Thanks
Peter