In general, strings do matter. New strings will sound brighter, have more sustain, sound fuller, will have more "snap", etc. There are different types of strings, and different ways that strings can be wound, and it all makes a difference. The differences might be subtle, and you may not always notice a significant difference if the rest of your rig isn't putting out a decent tone.
Same thing with pickups and EQ. Actually, I'd say that it's all about EQ. Every part of your rig eq's the sound, at least a little... everything from the cable to the type of wood in your guitar to the speakers, etc. Everything contributes. Good pickups will make a huge difference, but only if your amp/speakers don't suck.
In my personal opinion, priority goes something like this :
amp/speakers > pickups > pedals > guitar body > cables
Everyone will disagree with me on this, but I'm specifically thinking of distortion when I make that list.
What I'm trying to say is that if your amp sucks, then you'll hear a difference with different pickups, but you probably won't hear any difference at all with better cables. Priorities! If you can upgrade just one part of your gear, it should be the amp or speakers, because they make the single biggest contribution to your tone.
Knowing how to use EQ to your advantage is one of those voodoo black-box things that most guitarists don't get but make such a huge difference!
3. Wood + cavities = acoustic resonance (ie an "acoustic" sound). So a fully hollow guitar with a large body will have a fuller and more "acoustic" tone than a semi-hollowbody guitar or a solidbody.
Never heard of 'Hertz'.
I personally love Takamine guitars, and their generic brand, which I *think* is called Lotus, something like that. I have a Fender acoustic, and that's pretty decent too.