1. what does a P-90 sound like, how is it different than a humbucker?

2. if i were to put some gibson pickups in my epiphone would it actually sound that much better, or do I have to buy a gibson guitar?
Cream Schecter Ultra III
Black Epi Les Paul Studio
Hohner Red SG (yeah weird)
Breedlove Dreadnought acoustic
An old fender (unknown model)
A frankenstein strat (unknown model)
15w Vox Pathfinder
120w Crate Flexwave
1. A P-90 is a single coil pickup, so it's a lot mellower.
2. Yes.
A p90 is brighter than a humbucker, but thicker than a single coil. It is actually a big single coil, in terms of construction. It has a lot of "bark" to it.

Depends on which epiphone. It may sound better, but it won't be as good as the real thing in terms of quality
well putting a gibson pickups sounds like a good idea

after all pickups make a really big diference in the quality of the sound
My Gear

Squier VM p-bass(i chosed it over a fender!!!) with quarter pounder and gotoh 201!!
fender MIM P bass
epiphone SG 400
I agree it would sound better and also that it probably won't sound as good as a Gibson, though there are some exceptions to both. Gibson pickups are great but there are some cheaper options that are just as good or better and sound very similar.