Need a little help or an opinion NZXT Guardian 921

All those are 240mm rad units, and your case doesn't not support dual 120mm fans so there's nowhere to mount it.
However, it does look like there is physically the room for it. so if you drill your own holes and mesh at the top you could mount a unit like the H100i.
Those 120mm rad units should mount without needing modification.

As for which one to get, TBH the answer is none of them.
CLC (Closed Cool Coolers) units dont perform as well as you might think, they are at best on par with equivalent air heatsinks, often for a much higher price. For instance the H100i is roughly equal with a Noctua NH-D14 for performance and costs $30 more. For good performance with water-cooling, you want to go custom.

Cheapest way to get a custom loop off the ground is through the use of pre-configured kits like this. The lower end ones offer very good value for what you get.
http://www.frozencpu.com/products/16891/ex-wat-219/XSPC_Raystorm_750_RS240_Extreme_Universal_CPU_Water_Cooling_Kit_New_Rev_4_Pump_Included_w_Free_Dead-Water.html?tl=g30c321s1310
Just add your own coolant (Distilled is fairly cheap from any supermarket), and you'v got yourself a CPU loop.

However, if all your playing with is ~$100, wouldn't go custom. You just simply need to drop more money into it. That kit above is the minimum I recommend you get, and after adding in water that's $150.

Havent looked up reviews on that Zalman unit, so I cant really comment on it. But if it performs similarly to every other single rad CLC on the market, wouldn't expect anything great. Better off going air-cooling.
http://www.newegg.com/Product/Product.aspx?Item=N82E16835608018

Poland spring?
I'm guessing its a bottled water brand?
If it says Distilled or Demineralized on the bottle, go ahead. If its Mineral water, don't touch it. The two are completely opposite in intent, one has gone to pains to remove metals and such from the water, the other has deliberately added minerals/metals and such.

If you want colour in the loop, tubing and lighting effects is the way to do it. Dyes and such will eventually clog up your loop and stain tubing.
How much Dead Water to put in will say on the bottle, but basically yeah. Recommended you change the water every 6 months or so, but its not essential with a loop running plain distilled.
Maintenance will depend on what you do with the loop. Run a dye and you can expect to be constantly changing the coolant, cleaning blocks and replacing tubing. Set it up right the loop right, and maintenance can be as simple as occasionally draining/filling the loop and dusting the rad.

This is what I said in another thread.
Whether its worth adding water to the computer is up to what you want.
- If your after pure price/performance, stick with air cooling. Water-cooling costs too much for it to really compete here.
- If your after outright performance, then water-cooling is the way to go, especially when (its inevitable) you start on the GPU's.
- If your after a way to make the rig look and be awesome, custom water has got no competition here.
- If you want to make the rig unique and sort of "further" yourself as an enthusiast, then its one of the best ways to do this. Many here will tell you water-cooling is more of a hobby than anything else.

If your interested in Custom water-cooling, I suggest your read through the water-cooling sticky (multiple times). It goes over the the info you need to know and the explains the core concepts behind water-cooling.
http://www.tomshardware.com/forum/277130-29-read-first-watercooling-sticky

The Noctua NH-D14 is up there at the top tier of air coolers. The Thermaltake Frio and Phanteks PH-TC14PE compete against it, and depending on which review you look one will be better than the other and so on. IMO its too little a difference to really make a big fuss over, any of those will serve you as good as air-cooling gets.

Noctua make the best fans, just outright. You don't want to replace them with a fan that comes stock with your case.

Far as I'm aware, AM3 and AM3+ are the exact same socket except AM3+ has extra pins on the inside, same mounting mechanism I believe.

Closed Loop Water-Coolers (H100 type stuff) perform as well as high end air coolers, a custom loop will easily outperform CLC's and big air.

Closed loops arent designed to be opened up (With the exception of the Swiftech H220), and the people that have weren't very happy with the results when they did.

Peltiers are actually pretty cheap, though the rigging needed to use them effectively isn't :lol:.
Its how you chill water BTW, though somewhat experimental technology when it comes to cooling PC's.

TBH, get whichever one you want. You'l be fine for cooling regardless of which one you go for.