PC vs. MAC
I'm not a big fan of Macs. But that's because I grew up with Windows, and my only exposure to Macs is through my school, which uses the bottom the line iMacs. And I have to say that those computers crash as much as my old PC, and more often than my new one. Mind you, a good deal of that is because they're connected to a server, and it's a crappy server.
The old Macs were decent machines, but the new ones are absolutely brilliant, assuming you have the wallet to support it. A brand-new G5 Desktop will set you back about $10 000 (Canadian), but for that you do get 8 GB of RAM and 500 GB of harddrive space... I'm sure the equivilent PC would be the same or more.
As for OS... everything before OSX is useless. At the moment, though, there's not a lot of software available for OSX - my school still runs OS9 because we can't get some of the software we need for X. But, for the purposes of this discussion, Final Cut Pro runs on OSX, and I guess that's the most important?
In the end, it really comes down to what you feel more comfortable with. A good PC won't crash, and antivirus software protects you from the internet. I'm more comfortable editing on a PC - I've been using Adobe Premier 6.5 and just recently got Pro.
You can edit with 6.5 on a Mac, but if you want to use Pro you'll need a PC (or install Windows on your Mac, which is possible for some peverse reason). Likewise, if you want to edit with Final Cut you'll need a Mac.
Really, it depends on which OS you feel better with, and which program you prefer... though if you like Premier, I'd suggest a PC, since they stopped releasing it to Mac after 6.5.
Oh, editted to say this: Since you're asking about Laptop vs. PC as well...
Laptops are great. They're mobile, they're small, and they're dead useful. But they're not great for video editting. Generally, they have smaller screens, smaller harddrives, and less RAM. You'll need an external harddrive for sure, and you'd probably want a mouse and keyboard, and maybe even an external, larger, monitor.
Which isn't to say that it's not worth it - all that allows you to edit perfectly well at home, and even do a little on the road. But for your money, you can get a very powerful PC or an average laptop... and I'd rather have the PC.