You're paying for integrated design and stability. I have no problem with paying a premium to have access to OSX - it's a wonderful OS with features Windows just never thought of.
And I very much doubt any company can match Apple in packaging or design, I'd be wary of Sony products right now - their profits dropped 94% last year, and they're axing a lot of their departments (computers aren't likely, but you never know).
Yeah you'll probably need 1gb of RAM to use Aperture smoothly, I have 2gb because I got a student discount. 
To all the people on the fence - if you buy a mac, you will enjoy using the computer. I can guarantee that. It doesn't have multiple versions that remove features or add others, you get things out of the box that Windows has to cover with third party applications.
I'm using a 20" 2.16ghz Core 2 Duo iMac, 2gb of RAM, 256mb ATI card, 250gb HD (with 500gb external). It is blindingly fast, and churns through any program you throw at it. Aperture is beautiful, and very intuitive to use.
I will say that Parallels is quite good, but useless if you want to play games or do graphically heavy work. Install Bootcamp instead and have a small windows partition, and it'll run fine. It's actually easier to install Windows on a mac than on a PC - honestly.
For those switching, here's a list of nice applications that are either cheap or free (which is the great thing about OSX, much of the useful stuff is cheap):
Firefox (duh!)
Camino (basically firefox optimised for mac)
Newsfire (RSS reader, very nicely designed)
Thunderbird (alternative to Mail, both are good)
Adium (messenger program)
Toast (bit more expensive, but a great CD/DVD writer)
Many people will probably want a word processor too, there are some great free ones on the internet, like Open Office. But I will say that iWork and Office for Mac are both very good too, although iWork leaves out complex controls for better designs.
Hope that helps a bit!
Now, go buy yourself a mac, and you won't regret it. Ever noticed how every Mac user touts how great they are, but Windows users are either looking to switch, or mock Mac users?
It's because they can't hold shift and minimise applications in slow motion 
Oh yeah, and for those who don't know - hold ctrl and use your scroll wheel to zoom in or out (very smoothly) - it's great for those with bad sight.