replacing an alternator, power problems

right now i have 2 infinity 1252w wired at 2 ohms with a profile 1200 running them at 300 rms a piece, i have replaced my "big 3", and my lights are still dimming, i planned on adding a second battery, i got the pac200 battery isolator or relay whatever, then i heard that if my alternator doesnt put out enough its not going to matter anyway, so i got a few questions, first how much should an alternator put out to run two batteries, second how do i go about getting that checked out,
oh yea, and the reason why i want to take care of this problem is because i am adding two more infinity 1252's and going to run those with the alpine mrd-m1005, so there is no point in doing that if i cant power it all, any thoughts or ideas would be helpful

 

not sure how big it is now, ive never had to do anything with it, its in a 2000 chevy blazer if that helps out any

 

i have all 4 gauge stinger wire
let me run the big 3 by you all and make sure i did it right
1. positive battery terminal to alternator
2. negative battery terminal to engine block
3. negative battery terminal to frame

its a pretty new battery, probably not 3 three months old, its from a farm and fleet store, probably my weak point would be my amp ground, i drilled a hole into the sheet metal under my rear seat and have it held on to it with a nut and bolt, thats the only thing i can think of, maybe i need to go directly to the frame, or i did my big 3 wrong, i have no idea, help me out guys i would appreciate it

 

so i did that right, what do you think about my ground, that should be fine i would think, but thats why im so confused about my lights dimming, my electrical system should be able to handle what i have i think, but then what do i know, im stuck