Currently I am driving a 1999 infiniti g20 with justttt about 100,000 miles on it. All stock but audio and rims

I have the audio stuff installed..(check the sig)

and I plan to add more to the system later... once you start you can't stop!!
Thinkn about screen ps2 backupcam dvd.. so on

My question is... what do you think the next step should be
What should I do about my alternator/ battery?
I know I'm already dimming at night which is an obvious with the power im running on stock stuff, but I gota be pulling to much for my old car

Should I go to a yellow top with the stock alternator... Aftermarket alternator on stock battery or wait till i have money to replace both

Any and all suggestions would be great!
Thanks in advance!!