What gauge wire for my new amp?

I bought a 2000 Watt amp thag puts out 500 W RMS x 2 @ 4 ohms for my two Rockford fosgate P2s that are 400 W RMS. The amp case with an 8 gauge wire kit, but I was told I might want to upgrade the wire. Can I use the 8 gauge, or would it be a bad idea? Kinda tight on money.. lol
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