He,
we have the iMac 24" 2.8 ghz, 4 Gb RAM, 500 GB hard drive and NVidea 8800 GS.
Final Cut Studio is a lot faster than iMovie. But Motion 3 is very slow. So yes, it could be the software. I experience the same. (iMovie is slow with big projects, final cut pro is blazing fast in that perspective). But FCP is a lot less user friendly. Guess that's where the resources for iMovie go 🙂.
What you also could do is to collect all your important data from the hard drive, store it on a second hard drive and reinstall mac os x and all your programs. That will make your mac a lot faster. To be honest, we installed so much junk on our iMac (we really need to reinstall it), that my latest macbook pro basic version (only the hard drive is bigger) loads things faster. But it only has the necessary things installed: iwork, final cut studio, photoshop CS5. Nothing more. (ok, a few extra programs like airvideo and transmission).