ok so i have a simple question. i have 2 guitar necks. so all i have to do is unscrew the neck on the guitar and attach the new one on and it will work right?
Not necesarrily.
While it may well just bolt right in there are several things to look at.
1- They need to be the same scale or your intonation will be off (that can be fixed)
2-The neck pocket on different manufacturers(even some in the same brand as well) can be different Some are square cut some are rounded. some are 2 1/8 some are 2 1/4
3- The thickness of the heel . It may sit higher up on the body so you would have to adjust your bridge height ( again this will mess up your intonation.
Ther are a few other things that I just can't think of at the moment but you get the idea.
Now you could get lucky and things will work out tickety boo. I have had that happen a couple of times where they were easily interchangeable but I have had others where they requried shimming to raise them.and shimming so it would fit a wider pocket. I have also had to widen one pocket.
Give it a try though. Worse comes to worse just switch it back for the other.
pics of gear updated on profile 11/16/09
well i have a peavey strat and i have a fender strat neck. the fender is 24 frets while the peavey neck is 21
fender doesn't make 24 fret necks.
coming soon... parker fly mojo flame
ibanezes: rg350dx frankenstein * rg7620 7-string
fenders: telecaster plus * stratocaster plus

line 6: pod xt live * ax2 212
other: laney vh100r * mesa dual rectifier * monster cables
are you sure? something is telling me not to believe my friend.
screw that...

anyways...... if i want a 24 fret neck for my peavey strat, where can i get it?
does anyone know any websites (not ebay) to buy a guitar neck?
The neck isn't the problem - it's scale length that needs to be considered. The fender neck might fit fine but the frets will basically be in the wrong places for the notes they're supposed to play! This intonation will be far too much to be corrected normally, so you'd have to move the bridge.

The only thing I can think of is to fit the fender neck (incidentally, Fender do make 24 fret necks for a few less popular models,and warmoth make them for strats too) and then move the bridge further from the neck. Unless you're a pro at routing and have a lot of time and money to spare, I'd suggest buying a combined fixed bridge/tailpiece and fitting that. You'd have to be spot-on with your measuring too.
You would be far better off just selling what you have and try finding a good used guitar with a 24 fret neck on it.
With what you are trying to do you will have endless amounts of headaches and come up with a few new words for the english language in the process.
If you really like the Peavey then keep it the way it is and save up for something else.
Also in the future you should include as many details in your first post as you can like the fact that one neck is 24 fret + one is 21 fret.
Then the first response you get will be more accurate and alot shorter.
pics of gear updated on profile 11/16/09
websites you can buy necks are mightymite, warmoth, musikraft to name a few... mightmite would be the cheapest alternative for a neck, i would go that route as warmoth is big $$$... go warmoth only if you want exotic woods for example.