how about if i bought a brand new 15'' macbook pro?
surely its better than my crappy 15'' HP 4gb 250gb i have now?
I would say so yes, but I'm a bit of an Apple fan so take what I say as you wish. It's all down to preference of OS and software.
I've just bought an i5 15". Got it for £1285 off ebay, brand new, sealed with Applecare. For £500 cheaper than Apple, I can live with the no pound sign and small return key as it's a US model.
I upgraded the RAM to 8Gb and put a faster drive in it, runs Aperture 3 pretty well now. Not 12 core Mac Pro speed but much better than the Mac mini I was using before.
Regarding the iMac, if it's a G5, that's a bit more than I would pay, even though that would be a ~2 grand machine when it was new. I'd say £200-250 for that one now. The biggest problem with those machines is the power supply capacitors. Some were sourced from companies that stole an incomplete formula from Phillips, if they blow the machine is pretty much knackered unless you're really good with a soldering iron. Mine still works fine though, and is great for my Mum to use for iPhoto, Internet and email etc until it eventually packs in. My mates bought at the same time died a few months ago - Apple quoted £600 for repair, so it got sold on ebay for around £50
Bear in mind as others have said that a G5 iMac won't go beyond Leopard 10.5 or beyond Aperture 2.