Cheers Martin,
Let me suggest an informed strategy here:
1) Gigastudio does NOT play well with other toys. Forget trying to run Gigastudio at the same time as a VSTi or anything else for that matter. If you want to run Gigastudio, put it on its own computer. Trust me on this.
2) You want redundancies in a live situation. Think like this: "If this one thing breaks, am I totally lost for the show?". Another reason to get a second computer.
You should be looking at a distributed system where the responsibilities and loads are spread across two computers, rather than trying to build the "ultimate" computer and having it do everything.
For portability, you can look into laptops. But also look into Small Form Factor (SFF) pc's. these things are very very small.
look here for a link:
http://compreviews.about.com/cs/desk...psffpckits.htm
although not amazing for sound quality, a *really* reliable soundcard is the m-audio audiophile 2496. those things are dirt cheap now, and have excellent audio and midi response. i believe most small form factor computers give you one pci slot. that's just enough to get you going with the 2496 card. i don't think most club goers would notice if you were using an Apogee or the M-audio. Most club sound systems stink anyway, and it's surprising how little the average listener pays attention to details like this. Especially in a beer-drinking friday night bar situation.
when going for system reliability, remember this: the soundcard must get along with the mainboards chipset on the computer. not the cpu, the chipset. the chipset is what directs data traffic on the mainboard. Via chipsets used to have a bad rep many years ago, but i've seen excellent results with Via chipsets on more recent builds. Its all about "this mainboard, with this chipset, with this soundcard, at this point in time".
So if it were me, I would get two small form factor PC's that have chipsets that are friendly to the Audiophile 2496, and slap one of those cards in each and be done with it. Rather than buying the "ultimate" gigasampler giant version, i would get two smaller versions and load one on each computer (are you really going to try to play 256 voices at the same time live?). in fact, i would build each computer identically, so if you experience a system failure you can prioritize during the night on the one system that is still up and running. hard drive failures can occur at anytime. so don't let wishful thinking lure you into believing it will never happen to you at a live gig.
also, when you try to buy at the absolute top of the speed range, the price spikes way high. don't freak out thinking you need the ultimate latest processor. the stuff is so fast these days, that unless you're using a convolution reverb or something else way processor hungry, you should be fine.
also make sure you get a 7200rpm drive in each computer. the problem with laptops is they give you weak drives to try to save on power. with the small form factor, you can get a 7200rpm, perhaps a 10,000 sata one.
the completely pro thing would be to have 3 identical computers. two for live use, and a third one waiting in the wings for you to power up quickly in the event that one of your computers dies in the middle of a set. i would only bring two sets of monitor/mouse/keyboard. and if one of those failed, i would simply use a KVM switch to control both computers from the setup that is still working.