water pump change

if i was to change my stock water pump to a high flow, would it effect the hp? would i notice anything diff?

The difference would be minimal if any. I doubt you would be able to tell any difference.

whats the diff from the high flow, and oem?

I have never compared them < But it is most likely the size of the impellar (spelling ? ) or the number or pitch of the vains on it.

The high flow is just what is says, it moves more water. Basically the only improvements you will notice is better cooling. It wont increase the pressure of the cooling system, just like a high volume oil pump would do.