i have been a mac user for around 5 years now and will never look back to a windows based pc (well apart from i have to use one at work

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i have the iMac 24" 2.4 ghz, but they have just upgraded them to 2.8ghz, i bought mine from an apple store itself and got student discount which helped alot, i prefer to go into a shop/or online store (rather than Ebay) to buy large items like, mainly just because i know that if something happens i can always go back with it. also i noticed that the ebay one you found does NOT accept paypal, dont know if this is an issue or not, but if something goes wrong you might not be able to get your money back.
as for the photo editing side of things, i use photoshop cs3, which works beautifully on this machine. the more ram the better, (personally i bought my mac with 1Gb ram and then bought another 2gb from crucial which saved me a couple hundred pound compared to what apple charge)
all new apple macs come with iLife 08, which has a program called iPhoto, this allows you to arrange and store all your photos in certain groups etc.. it allows minor editing (red eye, crop,straighten, and adjustments i.e. saturation,contrast etc.) here is a link to the iphoto page and a video
iPhoto
also the operating system OSX. is just amazing, everything is just so slick,
another video for you, detailing the new operating system.
Guided Tour
also if there are programs that you need to use and they are only compatible with windows... no problem, you can use boot camp or parallels to run windows on your mac,( i dont personally)
hope that kinda helps a bit, keep us updated on your decision.
chris