Why is my 2006 iMac so slow now with Snow Leopard 10.6.8?

Question

Hello,
I bought and installed Snow Leopard on my 2006 iMac so that i could use my new Lacie Minimus Drives for backup using Time Machine.
Since my install though the computer is slow and everything seems to take a long time to do. For instance, when i select one of the shortcuts in my bookmark bar i put the mouse icon over it and it takes 5 seconds for it to go dark grey so i can select it.
It's like it has a time delay on everything now. Everything is sooooo slow.
Also before i updated the OS there was this sound coming off the computer like it was doing something in the background, a sort of rustling/crinkling noise from the drive i think. When it makes this sound the computer will slow down and display that little spinning disc icon until the sound went away and then it would speed up again. This was another reason i wanted to wipe my drive and update my software. That sound is almost constant now.
I wanted to wipe my entire hard drive and install the Snow Leopard on a fresh new system. I usually do this once a year to get rid of any unwanted junk on my computer but i never realised that the Snow Leopard disc doesn't give you the option of 'Erase and Install'.
So, do i need to install Tiger with the 'Erase and Install' option, creating a fresh system, then install Snow Leopard and update it?
Also would installing new RAM improve my mac's performance? (MichelPM, thanks for your suggestion)
There's one on ebay made by Sandisk for sale at £30. It's 2GB and seems like the best option. It's listed below.
I have 1GB on board now so that would make it 3GB.
Would doing this get rid of these unwanted and very annoying sounds and time delays?
If i have to hear that rustling or see that little coloured disc for much longer i am going to lose it.
Thanks for you time.
http://www.ebay.co.uk/itm/2GB-Ram-Memory-Apple-iMac-2-16GHz-Intel-Core-2-Duo-iMa c-2-4GHz-Intel-Core-2-Duo?item=260992423037&cmd=ViewItem&_trksid=p5197.m7&_trkpa rms=algo%3DLVI%26itu%3DUCI%26otn%3D5%26po%3DLVI%26ps%3D63%26clkid%3D899659199115 7919118#ht_4483wt_1165

Answer 1

Hello,
Thanks for the help everyone.
First, i have all my data/files backed up on my older Lacie drives. New ones are still in their boxes.
Second, the noise with the disc. It's not a clicking sound. It's more like the drive is struggling to do something.
It's not continuous, It happens when opening up itunes or iphoto or another app. Or when i try to open up a page on Safari, the page will start to load, the noise will start and either the noise stops and the page loads or the page stalls and the sound continues and then i just hit reload or quit Safari depending on how annoyed i am at the time! It did do this before i updated to SL but it seems to be more and more now!
If i have Safari, iPhoto and Mail open all at once, it's like a battle over what get's done with the sound creaking away and the spinning icon all over the place.
How do i check my drive using Disk Utility? Do i 'verify' it? I have done it before but it was a long time ago.
I never did a clean install. I just put in SL and let it update, then i updated the software to 10.6.8.
I will do that when i get the new RAM however just to start afresh.
I have bought a 2GB Crucial RAM memory (Thanks RadiationMac) but do i need to buy another one now?
Also how would spotlight make noise?
Thank you. These forums are invaluable.