"Since Macs can't be upgraded to include more hard disk space, you may run into a bit of a problem."
For what it's worth, this statement is untrue. Mac laptops, like any laptops, can be upgraded with bigger hard drives. There are up to 160GB drives available for Powerbook G4 models, and up to 200GB drives available for MacBook Pro models. MacBooks can also be upgraded similarly.
The installation appears to be fairly simple, based on this link (but be advised, doing this yourself might void your AppleCare Warranty):http://www.andybiggs.com/blog/?p=53
You can also get it done for you. In New York, there's Apple Authorized Service Center that installs customer supplied hard drives for U.S. $130. That's a reasonable price, so you should not pay more than that (or the equivalent, if you're not in the US.) There are a couple of places that you can ship to Mac laptop to for 24-hour upgrades for about $250-$300 (they suppy the drive.)
Another option would be to buy an external hard drive. 160GB, 200GB, 250GB - they make some pretty big ones these days, and they cost relatively little. A quick internet search should turn up a wide selection.
The big drawback between the MacBook and the MacBook Pro is the former's lack of dedicated video RAM. Read this link for a good side-by-side comparison:http://debris.com/journal/1452
Another MacBook drawback, in my opinion, is the glossy display. I hate glare on my screen, and glossy screens = lots of glare.
As for the "PC vs. Mac" debate: I have used both, and I personally prefer Macs for pretty much everything (except gaming, which I don't do much of so I can't really comment on that.) In general, I find Macs to be more reliable, with a cleaner interface. They have plenty of processing power and, if you have to, you can run Windows XP and Vista on any Mac with an Intel processor. So a current Mac is essentially two computers in one, which certainly doesn't suck.
I also find that, feature-for-feature, Macs and PCs are pretty close in terms of value. Macs seem to be so much more expensive because they come with a lot of standard equipment. Configuring a base PC to match a standard Mac would result in a more narrow price difference.
If you really want to stay within a reasonable budget, look into refurbished MacBook Pros at the Apple Store online. I recently saw a nice 2GHz dual core model for about $1399. That's only $400 more than a brand new MacBook ($300 more than a black MacBook). It might be worth a month of macaroni and cheese dinners to blow the extra cash on a better machine.
And from my personal experience, a refurb Mac is every bit as good as a new one, only a LOT less money.
Whatever you decide, good luck!