A powerful 4U ATX is what I have used w/ Scope DSP cards for years now. I always am looking for better solutions, but they all seem to be rackmounted also.
I view the V-Rack and Receptor Pro as an upgrade path for me in the future as opposed to going to a 64bit O.S. prematurely.
Using 4GB's of DDR2-800 on an Intel DP35DP w/ 3 x Scope Pro DSP cards hosted by Bidule and the 3GB PAE Switch has been plenty of stable live power for me. Only when I try and use SISS & VSL together did I see any trouble w/ memory addressing.
Using Modartt allowed me to eliminate a few large instruments, which in turned allowed SISS & VSL to loaded together w/ other K2/GVI based instruments.
But I do not use both live but was happy to see that I could use a Physically Modelled instrument that gave me more free RAM while providing similar sounding libraries.
I do have a new option after a decade of Scope/Giga 4U DAW's with the release of a new Scope 1U DSP rack called XITE-1.
It appears as though I can use this to not only replace the 3 x card 4U DAW, but have 350 % more power.
Having a shock-mounted 4U DAW w/ a Soniccore A16U converter, a 3U Analog Synth, SurgeX 1U, XITE-1 1U, and a Lexicon PCM91 AES/EBU connected 1U, a MIDI Solutions F8 pedal controller, all controlled by a 17" laptop w/ external HDD's via PCI-e eSATA 2 x port card looks to be a very powerful mobile recording/live performance DAW.
I recently went from 28U, to 16U and by summer '09 might be using a 4U and a laptop.
I will be broke but can go anywhere, anytime, no crew, no having some Union stagehands moving my rig around.....
I have upgraded from the 2GB DDR2-800 w/ the E6850, to the E8600 w/ 4GB's of DDR and while being able to load more VST's, I noticed no difference w/ the faster Core 2 Duo.
The laptop I am looking to build will use the T9600 CPU and will be plenty powerful. I still think a fast Core 2 Duo will work fine for along time instead of the Quads.
Since I use DSP based modulation & mastering effects and hardware reverb the need for all of these Octo Quad Xeons, etc. thankfully isn't necessary. I always disable that stuff except for pedal down IR's which sound better mixed w/ hardware reverb anyway.
Next year I look forward to better developed 32bit apps and a smaller rig.
64bit just doesn't seem necessary when there are so many other ways to enhance an already stable set up.
I hope someday to be able to have a Linux based rig and get rid of the silly debacle that M$ & Apple seem to keep playing.
Just imagine how nice life would be w/ a real O.S. made for audio.
Now I am really dreaming.
Happy New Year.
Hardware Analog, DSP, PhysMod. VSTi Romplers....