I think you need to re-direct your question on how to solve your problem Vs upgrading the hardware. Clearly you spent the effort on getting the max RAM and the i5 is not a slouch of a CPU! With the proper care & feeding your system should be singing!
Systems can bog down for many reasons not having enough RAM or HD space is the most common. Here you have enough RAM, but do you have enough free space on your HD and have you been good about defragmenting it. Over the course of time the drive gets fragmented while the Apps & Data can get into trouble the OS it's self does as well (even if you have a lot of RAM).
I would start off creating a external boot up drive HD or thumb drive so you can boot up under it. I would also make sure I have a good backup of what I currently have on my internal HD before I proceed to the next step here
Next you want to boot up under the external drive and run Apples' Disk Utility program to repair the drive and to repair permissions. See if that solves things first.
If repairing the disk with Disk utility didn't help you'll need to get a defragmentation program, I would recommend getting Drive Genus there are others which can be used as well. Using it you should be able to see how bad your disk is fragmented and can de-freagment the drive. That should make a big improvement.
If the drive is not badly fragmented you may have some apps running in background that are misbehaving. Use Apples' Activity Monitor to see what is chewing up all of the CPU cycles. Google the files name to see what it is or if you can identify what app it belongs to.
Of course I assume you have made sure you have maintained your OS & apps here making sure you have them updated with the latest version and/or updates. That you have a good antivirus app running and have the latest definition files for it. If not you may want to start here.