What should one know when changing from PC to Mac for the first time?
Well #1, if you want a decent mac, be prepared to spend at least $2,700. It's infuriating because you could get a PC with the same specs (or maybe better) for $800. #2, be prepared to replace almost all of your programs, for most of them won't work under the mac OS operating system. You could run a PC emulator on your mac, but I'm not gonna lie......it sucks. You'd have to dual boot with XP. Sure that solves your problems now, but no one has been able to dual boot mac OS and Vista. What makes you think they'll be able to dual boot with windows 7? Macs just aren't very powerful when you compare them to PC's. They really don't need to be. Their OS takes up about 1/10 of the resources than MS's OS (I'm going off of practical usage, NOT what each OS says on the box since MS lies). Your best solution is to replace all your current programs with programs that work on the mac OS, which is expensive and not very practical. In terms of piracy.....well mac programs don't have hardly any protection at all against it (except for a few major programs such as photoshop and other programs that work on both PC's and macs). In order to switch to a mac, you must be willing to spend A LOT of money and replace some of your programs for ones that might not work as well. If you've used PC's for over 3 years, chances are the switch to mac will be painful. Same goes for mac users. It's just what you've grown up with. Changing it is very hard.
What are good specs for an iMac?
I'd say you'd need at least a core2duo 2.4ghz processor, 2gb of ram, and a 200gb hard drive to use a mac effectively with 10.5. If you're gonna be gouged by a mac's high prices, you might as well go all out though and get better.
Which PC programs work on Mac?
As I stated above, you can dual boot or run an emulator, but only major programs run on both macs and PC's.
What's the differences between the Mac OS's?
HUGE. Go with 10.5. It's got a billion more things than 10.4. Even the versions 10.4.2 and 10.4.8 have significant differences. Go with the best. When a new OS or a new version of the OS comes out, upgrade immediately.
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To conclude this lengthy questionnaire, I would say to get a decent mac brand new and to get all the programs and accessories you'd want, have at least $5,000. Get one used and use pirated software (both of these options I would suggest against) and spend at least $1,500. Don't have that much money to throw away on a mac? Don't even try to go cheapo. Wait until you've got the money and then spend it on a mac.
Oh, and here's an example of the MINIMUM I would buy:
http://minneapolis.craigslist.org/sys/921567767.html