I'm running 1000 watts RMS (500 to each sub)
I have the yellow top OPTIMA battery & I notice no dimming on my lights at all...

I always upgrade the battery or recommend it... since your only pushing 1000 watts I see upgrading the alternator as unnecessary

 

Not sure of a Chevy HO alternator, but Mechman makes them from the mid $200's on up depending on build need. I run his S Series in my Monte, I think it was about $500.

Its better to go with a reputable builder though, rather than adding multiple stock alternators, or having a rewound stock.
They're phased so they deliver a scaled given output at a given RPM like you mentioned. Simply getting a rewound or extra alt doesn't necessarily give you the boos in the RPM range you need like these would.
I've pretty much had them all and its laughable now that I have the Mechman.

There are other builders out there, Steve Meade forums are a good place to find specialty manufacturers for things like alternators or batteries, sound proofing, etc.

I would still only recommend you going with a good single battery upgrade though. Best place to put any cash on your electrical at this point, think of it as a foundation.