keyplayer,
I don't own or use giga studio, but there are a few things I can answer for you.
As to your first question, yes and no. Take Trilogy for example. There are about 3 gigs worth of samples in that thing. A 5080 or Triton has, I'm guessing about 128MB's, maybe 256 MB's. And that's for ALL their sounds, not just the basses. Trilogy also gives you performance oriented samples such as slides, X-notes, thumps, etc... which when programmed well, helps create a much more realistic sounding bass track than was ever possible using "Picked Bass 1" from the ROMplers you mentioned.
Trilogy is a VSTi. It has it's own interface, and though you can edit the programs somewhat from the interface, you cannot edit the samples themselves. You cannot add or delete from it's "core" library. As far as I'm aware, you need to have a DAW platform to run this type of VSTi. This plug-in won't run on giga studio. Someone please correct me if I'm mistaken.
Giga Studio is a software sampler, and is a stand alone program. You can import your own wave or aiff files, and create your own instruments from scratch if you wish. A more common approach is to purchase sample libraries already formatted for giga studio. There are hundreds and hundreds of excellent libraries already in existence. Software samplers like giga studio, Kontact, EXS, Mach 5, etc... have become popular because they stream samples from the disk drive. Looping notes so that they sustain properly is no longer a necessity. There are piano libraries in existence right now that have all 88 notes of the piano sampled until the sound of each note decays naturally. Each note in some cases was sampled at 8 to 16 velocity levels, with and without the sustain pedal down! Obviously, without the limitations that were common place with hardware samplers, developers are taking advantage of this, and creating stunning libraries with gigabytes and gigabytes of samples that include practically every note and articulation available on any given instrument.
As for the separate PC to run these things, that's fairly common. I myself use a Mac, a dual 2gig G5, and I run everything in Logic. I typically can get roughly 20 to 25 EXS's (that's Logic's soft sampler), 2 Stylus VSTi's, Trliogy, and 5 or 6 other VSTi's before my computer flips me the finger!
If you had separate dedicated machines, I'm sure you'd get more milage than that.
Hope that helps...