Thoughts:

-First of all it all comes down to budget....

Video Card: Matrox line of cards are the best for dual displays.
Their software and drivers rock for adjusting the dual display capabilities.
I have the P750 (which is a bit expensive) running 3 displays. It rocks.
For 2 displays you can use get away with their older line of G-cards. G550
or G450... (Both are really cheap now and are considered the standard
by many for dual displays users).. .... All that said, if you want to use you
computer for gaming as well as audio, I'd stay away from the Matrox
cards. They are not gaming cards. Many cards now days have dual
display capabilities and work for gaming (NVIDIA ir ATI..wide range
depending on $)..

Hard Drive: Consider the new Raptor SATA drives from Western
digital. They are 10,000 rpm and the fastest that there is. Capacities are
36gb and 72 gb and are at least twice the price of 7200 rpm drives.. but
the performance is killer right to the end of the media. (Don't fret if that’s
out of the budget though.. 7200rpm will make do)

Sound Card: Everyone will have an opinion on this.. but RME 9632
is the reference for low latency. If 8 i/o is needed then there are tons of
options. (M-audio, Emu,.... ).. . Better to get an internal PCI based card
with a breakout box if you are building a pc. (Firewire is an overpriced
Mac based creation)

Add-ons: If you really want to make an audio computer.. do it
right, sell your mothers stove and get a UAD-1 card.

**Good job building a PC rather than getting a mac.. It will be cheaper,
you will get more, there is more software.. .. and I don't care what
anyone sais... as long as its configured right and you did not throw any
crap parts in there... a 3ghz, 800fsb PC will smoke the mac.**

Building your own pc=Techno
Buying a overprice MAC=Yuppy