Here's the thing:
I've got a Peavey Valveking wich I run on almost maximum distortion. Some of my older guitars make a hell of a lot noise when I'm not playing and some other weird sounds when I'm playing. I'm thinking of getting a noise gate, but will that solve my problem?
I live in the Netherlands so the ISP Deciminator is hard to get by, will the NS2 or the MXR smart gate do the works?

First, I'd try turning the gain down a bit...I know if you're trying to get metal out of a VK its going to be hard to turn it down too much. I couldn't live without a boost with my old VK...and it was nice to have an EQ pedal in the loop too. But, if you're happy with the gain and sound you're getting, a noise gate/suppressor might be just what you need. I bought a NS2 when my ISP went out on me, and it does the job just fine. They're a lot cheaper and you can use the X-pattern to run it out front of your amp and in your loop...and you'd have to buy the g-string version of the ISP to get that.
MXR smart gate should be pretty good if you can't get the ISP. If your NOT going to be switching to a clean channel, just put the gate in the loop.
If you do switch channels, you'll either want to get a pedal that you can do the X pattern with (guitar>gate>amp>loop send>gate>loop return) or 2 gates one for the loop (kept VERY light) and one in front of the amp to gate the guitar.

More gain=more noise. Put an OD in front of your amp, and your gonna here more noise as well. With high gain, if you want silence, you need a gate. It doesn't matter if you have perfect power, run active pick-ups and turn off all the lights, your gonna have hiss from the pre-amp.
Ibanez RGR421EXFM, Michael Kelley Vex NV, Ovation Celebrity. Carvin V3
Peavey 412M w/Eminence Wizards & Swamp Thangs, Rocktron Hush Super C, Furman PL-8C,15 band EQ, Boosta grande, ISP Decimator, Dano EQ, Ibanez TBX 150,TC Elec Polytune