why are my mac's always slower than my neighbour's pc?

[nino]

I've been using Macs for about 15 years now, OS X for about 12 months. I've always been happy to pay a bit extra and buy a reliable and aesthetically pleasing Mac than skimp and get a cheap and nasty pc. I loved OS 9, and now that I'm used to OS X, I love that too. Most Mac software is terrific, and iPhoto, iTunes, iCal, Mail and many of the other bits and pieces that come with OS X are works of art. The only thing that really disappoints me about my Macs is that they always seem to be a whole lot slower than pc's. I have an 800 MHz G4 iMac, and with OS X going it chugs along nicely, but today I got hold of a Dell 2 GHz machine and that really seems to fly. Acrobat Reader opens in less than a second on the Dell, it takes several on the iMac. New document windows flash open on the Dell, on the iMac they seem to just glide open. Web-browsing ALWAYS seems HEAPS faster on the PC's around my workplace than on any macs I've ever had. I used to think that a Mac with a given clock speed would be faster than a pc with a similar or somewhat higher speed, but that never seems to be the case.

So, my questions...

Am I expecting too much from Macs?

Should I be surprised that the 2 GHz Dell is faster than an 800 MHz iMac?

Can I expect my iMac to do ANYTHING faster than the Dell?

My workplace is dominated by pc's, and my area is one of the few remaining bastions of Apple products. It would be nice if I could justify the extra expense of my Macs with something more than their external beauty.