Wiring gurus...

Wiring gurus...need help...I am upgrading my old 35A alternator to the 135A alternator that I just bought...it is marine...my question is...is 4 gauge wire fine to wire to the battery? Also do I need to upgrade the Circuit Breaker in the boat...it has a 40A Circuit Breaker? The boat is a 1986 Dixie Super SKier with a PCM 351W Motor...I am running about 1200W and only have 1 battery...I only plan on running the stereo during running the boat/wakeboarding...I have read some old discussions but I think it just confuses me more...any help or suggestions or even some websites that will help with the wiring...I am also thinking that maybe it might be a good idea for a 2nd battery...but funds are a little low...so just want to run it like it is for now...the circuit breaker just protects the dash components and motor? Unsure of what the circuit breaker is really for...there is not much electronics/computer in the boat? No EFI...it is carburetored...thanks for any help...

I'm assuming you bought the one wire alt? 4 gauage should be fine (how far is the alt to the battery) there are tables you can use to double check wire guage. it it makes you feel better 2 ga isn't much more....I used battery cable. I did not use a circuit breaker on mine. I have all my amps fused at a distribution block and the battery is the only thing that takes the full amperage. Have you had your alt bench tested. My guess is you'll proabbly see about 100 amps at 3k RPM (where you ride at) and 50 sih at idle??? but there are alot of guys on here that use resetable fuses. I was told that its not necc.