I dont' have the money, this is more of a quesion of curiocity, but could you use a PA setupONLY to run the Bass? Or do you HAVE to go with the far more expensive Bass Cabnets and bass heads?

If you are worryed about sound, I woudl use a Bass Preamp, or atleast a 31 band rackmount EQ.
Do you mean a PA instead of a bass amp?

You could, as long as the speakers can handle the frequencies and you have a large enough power amp to be heard.

But a PA usually doesn't colour your sound as nicely as an amp does, and a good PA isn't really cheaper than an amp.
"You can practice to attain knowledge, but you can't practice to attain wisdom." - Herbie Hancock
Whell yea^, but my biggest problem is finding a cheap 18" speaker for the sound I want.
I can go with this:
http://www.musiciansfriend.com/product/Ampeg-SVT18-Cabinet?sku=481840 (I woudl want this anyway if it was a bit cheaper)

Or this for half the cost:

Not only that, but it seems like I can get more power out of a PA in the long run than a Bas head or even a Bass PA.

As in 1200W
Up to 6400Ws (I think)
^ Would you ever need 6400W? And would you spend $4000 on a power amp that might not even sound as good as a bass amp?

Really, all the volume you'd ever need is enough to stay clean and clear over a drum kit. Any more and everything has to be mic'd anyway. That can be done with alot less than 1000w, let alone 6400w. And you'd have trouble finding cabs rated for that wattage, and then you'd have to lug them around. I think it's really for multiple speaker box setups.

I dunno about the peavey cab, you never now it might sound great. Or it might blow up on you.
"You can practice to attain knowledge, but you can't practice to attain wisdom." - Herbie Hancock
The PA will not colour your sound the way a bass amp will. Run it direct and all you'll hear is your instrument, otherwise put a pre-amp in front to make it sound more amp-like. Loads of bassists are using Sansamp Bass DI and a PA rather than traditional amps these days. You'll want to think about monitoring though.
^ hmm.. Actually that may work well with my low amp. concidering I really want a low powerfull sound.

Plus, I want a 6400W amp and a whole lot of 21" cabnets to push 20hz of audio bass overload! (yes it is my wet dream!)