Unless you have an ancient computer there is nothing to worry about.
You face not Malchezaar alone, but the legions he commands.
I think a more pressing issue would be WoW's current over-reliance on the CPU. I recently purchased myself a Geforce GTX460 (upgrading from a 9800GT), and the frame rate only improved slightly. With Cata reputedly supporting DX11, that will hopefully start changing for the better.
Mmm, as games get bigger so does the amount of storage on HDD's and SSD's. It's not really a question of if the game is going to be too big, but how annoying it is to re-install it when needed, due to new PC, crash or w/e.
And on average, with all content updates and such an expansion probably adds.. idk ~6-8GB total over what you alrdy have. Tops. So in the next few years, if it's still going strong, it will probably hit ~25GB or so.
Oh, later edit: I use and 80GB SSD, and a WD Raptor as storage/applications. It's a 2nd gen Intel, I've had it for ~11 months and I couldn't be happier, not even one hiccup or issue yet. I picked up a second one for my lappy... amazing boost in performance. Some poeple choose to adopt early, some do not. I like to adopt early because I know what I'm dealing with when they refine it.
You dont need a huge drive for your pc, myself i got 150gig for system files, rest of it is on an 2TB external drive, problem fixed
You should backup all your old patches onto a DVD/something else and then remove them from your HD if you are very concerned with the space.
As others have said, HD space is very cheap.
I'd be more concerned with hard drive failure than hard drive capacity, a lot of people never back up anything.
My entire WoW folder takes 25GB at the moment, and I can't imagine that going over 40gb with cataclysm.