People prefer using a mac for editing not only because a top of the range mac is much more powerful than say a top end Dell BUT because the Mac has a better OS for things like video and music related things.
the top end mac costs... wait for it... £16000!
You can however if you want a MAC but cant afford one build a Hack! hackintosh is a MAC OSX operating system running on a normal PC. exactly the same in every way, you can even get a mouse and keyboard.
a budget of about £1000 will see you with the perfect PC for video editing, Ram is quite cheap at the moment so i would even consider putting 12gb in, nice 120gb solid state drive and an i7.
If your budget is sub 500 quid though... i5 and 8gb of ram should get you started.
I love buying top notch computers!!
just spotted this, have a watch of the vid.http://www.hackintosh.com/#hackintosh_compatible