you will most prob need to add a 2 or 4 gauge power cable, with a fuse somewhere u need heavy duty. Also upgrade your earth as more power will be pushed tho. The size of speaker cable is up to you but the bigger the better !!!!!!!!
i got a 4gauge power cable kit with fuse, head unit connecters earth and a few other little bits for about £100, u wont need to change the headunit cables but if u do change the power cable you must change the earth.

My Hifonics 1500 amp has finally arrived from the states. Gonna get it wired up 2moz.
My current amp runs at about 400 watts rmsm but this is 1000 rms at 2 ohm. I asked before about the wiring, but, which parts will actually need replacing. I don't know the exact specs on my current wiring, but guess its pretty average.
With the increase in power, should i change the power and speaker cables, or just the speaker cabling?

Cheers