Photoshop CS4 running slow on MAC OS X

Question

I bought myself a MAC OS X recently with a student edition of Photoshop CS4, i am currently studying photography and have been told many times that a mac would be the best option to edit images- this coupled with Photoshop CS4. However since loading the program onto my MAC i have had nothing but stress it is being extremely slow.
For example when i start to add layers to my work it immediately slows down, esp when i start adding effects to any of the layers such as 'surface blur', the 'elliptical tool' is also slow as when i create a shape i see nothing come up on the actual image... For me to actually be able to see what i am doing i usually have to toggle between the icons on the tool bar on the side. Another slow feature is the cropping tool, when i want to move the crop the slightest bit by using the arrows on my keyboard the shape doesnt follow where its meant to go. What i have also noticed is that the 'preview' image doesn't apply on the image im working on, usually when an effect such as a blur is applied it shows what it would look like on the image before you click ok. At least this is my experience with Photoshop CS2 and a PC. This is all happening with just one image open, so you can imagine what it is like when i open multiple images... I will admit my images are large in size, but surely photoshop should be able to handle this? The macs in my university are able to handle this AND more (adding multiple layers and images on-top of each other) and they also use CS4.
I have troubleshooted it to death, even rung the store where i bought the program and MAC from and they have updated all the features on the mac and photoshop and this has not helped. Ive even checked if it could be the video card, my mac currently has the- ATI Radeon HD 2600 Pro. I believe this to be the most recent and up to date card however i haven't ruled it out as the source of the problem.  The ram has also been doubled and is now 4GB.
One thing i do notice is that at FIRST everything seems fine, and then when i really start to get into editing an image it slows down hugely and starts to show the problems mentioned before. I have just rung the MAC support line and they advised me to open a new account on the mac and load photoshop onto there to see if its any of the settings on my particular account, again everything seemed fine at first- but then it fell back into its bad habits of being slow. 
Any help or suggestions would be great as this is really getting me down, im a university student studying photography for my degree and i really want this resolved before i go back and start taking more photos.
Liz

Answer 1

The web psd files may be 100mb... not the actual files on a web site. 100mb files on the web would be silly All the actual final web images are almost always under 200k at the very most, more often ~50k max.
I do several different types of projects -- web development, print production, direct mail, advertising, illustration (mostly vector in AI though), and photography retouching. My working files always hit 100MB easy.. in print production or illustration is very easy to hit 700MB - 1GB with a file if not more. Final production files are rarely that large... But I retain all working files as well for easy editing in the future. Photography retouching doesn't often create such large files. That's not to say they can't... but it's just more rare with retouching since most of the work is spot retouching or overall tonal shifts.
File size is also greatly effected by how you work. The more non-destructive methods you use, the greater the file size can get. If you flatten layers file sizes are reduced. I don't ever flatten any layer if it can be avoided.
Forget OWC.. they're great but not the best for RAM.. 18004memory.com has the same ram.. the same warranty.. at better prices. 4x4GB sticks = $396 for 667mhz RAM and $463 for 800mhz both with free shipping. I've got 16GB of RAM in my MacPro from 18004memory and have been wonderfully happy for over a year with it. When I loaded my machine prices were easily triple those. But then.. the economy was much better then too.