You can get an aftermarket sound card, but the best thing to do would be invest in a ProTools HD system since those actually process data / plugins / audio "out of the box" that way no matter what hardware you are using you will be able to process fine.
That said the studio I work in uses dual 8 core Xeons in the main room, and Dual 4core xeons in our small room. And that is in edition to the ProTools HD rigs.
Also, due to the memory assigning properties of the Mac OS and the general stability of the architecture we were running the Mac OS. That doesn't mean you need to buy a mac, since you can install Mac OS on intel boards.
Hey, (ill start off w/ apologizing for the typing...this is on my phone)
I've built a few PC audio workstations.
1)what do u mean when u say "audio has to be top of the range"?
2) what software are you using for production?
3) what kind of music are you producing?
4) any outboard gear? MIDI or USB controllers?
I wouldn't invest in ProTools. Its expensive, REALLY expensive and most effects/ instrument processing can be handled by your cpu...its NOT "the best thing you can do" since we don't know WHAT IT IS you're going to be doing
Quadcores are ridiculously cheap these days (i7 2600k=$279 @ Microcenter for eg)..you won't "need" dual 8core setup anytime soon if ever...an i7 will do u fine
RAM is super cheap too...at least 8GB...just saw an add on SlickDeals for $30 8GB kit
I'm currently looking at boards now as I'm thinking of a new build
I'm upgrading from an Q9550 to an i7 2600 k...and that Q9550 still has some to go but I'm itching for a new system lol
I'm going with the Z68 mobo because it has a new intel ssd caching for hard drive writing...for me, hard drive writing and reading is important (for recording and accessing/ loading my sample /loop library files into projects)
...again though depending on what you doing, you may not need ssd caching and RaID configurations.
Z68 will most likely have dual HDMI out tho for your dual monitor setup if ur not investing in an aftermarket GPU
I'm looking into building a CustoMac but Win 7 PC would just fine also.
AFWIW, I use Ableton Live for DAW, Native Instruments Komplete and random VSTs, UAD plugins...if I build the CustoMac, ill get Logic (really the only reason I want to build a Mac OS run machine.)
Now about sound cards, or as you should start calling them, "audio interfaces"...the one suggested by a previous poster would be "ok" at best but ur just throwing money away because in 4 months you're gonna understand a few things about audio production and wished you had just put that money into something "just a little better"
In some aspects, you're building ur workstation around ur interface...main thing is how is it connecting to ur computer, to ur motherboard? USB? Firewire? PCI? And what will u be connecting to ur interface? Multiple mics? MIDI controllers? DAT? Monitors or Headphones or both?
What's your REALISTIC budget? And how much do you want to invest each month? (This will be an "investment" from here on out lol)