Hello:
I don't know if my set-up will help you but I recently added a new computer and had to address these same questions. I am running a set-up with 3 computers. All are Custom PCs from VisionDAW http://www.visiondaw.com/productcart/pc/index.asp and are optimized for their particular purpose.
1. PC (XP Pro, 4 GB RAM/3GB switch enabled) with Sibelius 5 and Sonar 8 PE for notation and sequencing. Also has Broadway Big Band.
1. PC (XP Pro, 4GB RAM no 3GB switch - GS doesn't like it) with GigaStudio Orchestra 3, VSL Pro Edition, Larry Seyer Drums, etc.
1. PC (the new one - see configuration below) with Kontakt 3 with, Sonivox Complete Symphonic Collection, the EWQL Piano library, etc
The newest is the one for Kontakt 3. The configuration is:
Intel Core 2 Duo E8500 - Dual Core 3.16 GHz
8GB 800MHz DDR2 (4x2GB)
WDC 160 GB Ultra ATA100, 7200rpm, 8MB buffer (system drive)
2x500 GB WDC SATA II 7200 RPM 16 MB Buffer - RAID 0) (samples drive)
Windows XP Pro 64
I don't know what motherboard was used. The rest is familiar stuff I'm sure (sound and video cards, DVD ROM, Various Interfaces, etc.)
Everything except BBB communicates via MIDIoverLAN. This works perfectly and invisibly. I have almost never had any trouble with it (other than getting it set up correctly). BBB reqires Maple MIDI which is free and also works perfectly.
You are probably aware that Tascam is no longer going to produce or support GigaStudio. I have Gigastudio 4 but have uninstalled it because I had so many problems using it. That could be my ignorance, of course. However, I tend to like Kontakt better and have much less trouble with it though I use both.
A couple of issues you might have to consider. One is whether GS and VI will run well on a 64 bit system. Also, XP 64 and Vista 64 are quite different so that might be an issue. I really don't know about this with GS so, someone else will have to answer it. I have had very few problems with the K3 on the 64 bit.
Another big thing will be to make sure you have a separate drive for your sample libraries (you may already know this but it wasn't mentioned in your set-up list). You don't want to have the sample libraries on the same drive as your system because this leads to a variety of issues. IMO, 7200 RPM is enough. There are others here who may have a different opinion and hopefully, they will share their thoughts.
Hope this helps.
Be Well,
Poppa