1. Do the big 3. It will make more difference than a tiny hc600.
2. The battery in your original link is larger than an hc600, so I don't know why anyone would recommend a hc600 over it.
3. Any battery around 20ah isn't going to make much difference. Iv'e got a 20ah battery in my trunk (stock 80 amp alt, big 3, and 1800 watts) and i'm getting some fairly large voltage drops.
all right, i'm lookin to buy a big 3 kit from someone on here anyway pretty soon, then i guess i'll buy the batt from batterymart for cheap and i should be good.Do the big 3 before you do anything else. Why not do it? It's easy, cheap, and will make a huge difference (most likely).
Don't know how big your alt is, but with 2k watts you're really pushing it no matter how many extra batteries you put in. Expect some pretty good voltage drops, though with big3 and batt you should be able to do okay.
Do not waste your money on that battery. Seriously. Waste of time and money. First you must do the Big3. Then HO alternator. Then a good main battery. Then you can add additional batteries. Optima, deka, or just deep cycle marine batteries.
The batt in your first post is a small motorcycle battery. Even my little 400 watt sub would pound the shit out of that batt in less than a minute. I wouldn't waste the time to wire it in. It adds nothing to your system.
If you budget is less than $100, spend it on wiring from http://www.knukonceptz.com/ ; wire, fuses and terminals.