Hey I'm thinking about getting a GReddy EVO exhaust and I am wondering if anyone knows how much HP increase to the wheels it produces?

3 maybe...exhaust doesn't give much hp at all, if you're after power get a header

i will go for a header, but not until i get the exhaust first. Do you think GReddy makes good exhaust? Who do you think makes the best?

yes the Greddy is very good, quiet but good

Other very good choices are:
Nismo
Stromung (this is what I would get if I had a qr25)
Thermal R&D
Vibrant
Mossy