Buying an iMac (starting at $1200 US$) isn't much more than a decently equipped PC, and assuming your comparing it against a similar vintage (current) PC setup, would require less memory and resources to do the same tasks. You also don't need the antivirus or spyware subscriptions. Don't forget to factor in the software pricing. Vegas Pro 9 is close to $600 US$, FCE is $199 US$.

Having used both, Vegas isn't even close to FCE. It is far better than it used to be, but they're no equals. The PC with Vegas has the advantage of getting you in the editing game for less money, but if you're talking about what I actually preferred working with more, my vote is FCE.

Don't use an external drive for additional editing storage -- USB2 or network storage are generally far slower than directly attached, internal, SATA or SAS storage. This will make a big difference for working with large files, so you may want to order that iMac with a large drive (or install one yourself afterwards).