Greetings Folks!

I'm an intermediate level guitarist and home studio hobbyist with a boatload of great gear including some great tube amps, speaker cabs, a few high-end rack-mounted FX processors, as well as a firewire-based mixing board to a Mac Pro with Garageband and Logic Express, but I'm completely new to recording, so I've got a few basic newbee questions for the membership here:

I prefer an ultra 'clean' sound on guitar about 98% of the time, and over the years, I've tried virtually EVERY configuration imaginable in an effort to polish my 'live' (home studio) guitar sound. This includes the use of premium FX (mostly reverb/delay) directly through the amp's FX loops, and even a few wet/dry/wet setups using the amp's Line Out to a solid state amp with separate cabs. However, no matter how I try, I can't seem to get some of the truly pure, seemingly multi-dimensional, almost 'crystalline' sounds that I see others achieving on YouTube and elsewhere using VST's.

The problem is, none of those sources ever seem to specify just how it is that they have their system(s) configured, and I won't belabor it by citing specific YouTube videos here at this early juncture, but suffice to say, they are videos of home studio guitarists just like me, using VST's (such as Waves Guitar or GTR3) to somehow process their 'live' sound with incredibly gorgeous-sounding results, and its the "somehow" of this equation that has me stumped.

Now, I realize that this is a deep subject that is FAR beyond the scope of a single introductory thread, especially when its been initiated by a complete "newb", but basically, here are the preliminary questions that I'm hoping to get some help with for starters (and please, forgive any seemingly ridiculous questions as stemming from sheer ignorance):

1) What specific VSTs (for Mac) are currently considered by the experts to be the very BEST for studio-grade guitar and what (if any) pitfalls or limitations do I need to watch-out for in purchasing them?

2) Can the FX provided by a VST/ plugin be active and heard while playing the instrument in real time? Or is it a situation where the FX can only be added to a previously recorded signal and restricted to playback?

3) I've seen reports of people using the Send jack of their amp's FX loop to send the dry signal to their computer/VST for processing, while apparently using a "dummy" cable (?) from the Return jack to load the amp and somehow disable the speaker(s) so that they can play silently with headphones while recording and still not damage their amp? But what is a "dummy" cable? And despite the use of such a cable on the peripheral FX loop, wouldn't the amp's dry signal still travel through to the cabs?

Thanks ahead-of-time to all of those who choose to contribute here.