Mid-2010 27" iMac with i5 - process for determine why slow

I have a mid-2010 27" iMac with an i5 processor and 12 GB of memory running latest Maverick OS. I mainly use the computer for email, web browsing, light weight spreadsheets and importing/organizing/sharing photos. I understand why my computer would be slow on startup (with reloading Safari pages, updating notifications, updating email, updating iMessage, etc.) but more and more often I see the spinning ball while using the computer. I have run though the basic steps of fixing permissions, etc. and would like to understand what is limiting my computer's performance. A few other things to note...

• I have 12 GB of memory (the original 2 GB x 2 and I've added 4 GB x 2). I have heard that mixing memory isn't best.
• My 1 Tb hard drive is at 80% capacity. I have heard that ideally this would be lower.

How do I determine what is causing my slowness? Is there a way of finding out which programs are demanding memory and by how much? I have read that my iMac can offically support 16 GB memory and unofficially support up to 32 GB. Not sure if 12 GB is a limiting factor or not. Just looking to get another year or so from this computer without getting frustrated watching the spinning ball. Would a fresh install of Marvericks and then reloading my personal stuff improve the situation?

Thanks in advance!