I use CuBase 5 as well with great results........ I built my machine with versatility AND value in mind.
I think the first thing that has to be decided on BEFORE any hardware is what operating system you plan to use AND what recording software??
I found that CuBase runs the most efficient on Win XP Pro so I dedicated one hard drive (WD 150g VelociRaptor) for nothing but Win XP and all my recording software (cubase, plugins, vst's)
Another VERY important part is to match you soundcard with your interface..... I use a Lexicon Omega interface....AWESOME interface, but it requires a soundcard that is ASIO 2.0 compliant....... I bought a Creative Soundblaster X-Fi titanium Pro...... I have near Zero latencies with it.
Are you planning on going all SATA?? (I hope so) then you will have to choose a motherboard that is either Sata3.0 or 6.0....your choice.
in a nutshell, my system works flawlessly for recording, gaming, and browsing. It was VERY economical because many of the components are out dated now......
My System specs:
MB- EVGA N-Force 750i SLI FTW (4 internal sata 3.0 ports....lame, but enough)
CPU- Intel QX9650 core2quad extreme (they're cheap now)
Ram- 4x2G Patriot Viper DDR2 PC2-8500 (1066mhz fsb)
HDD's- (3) WD VelociRaptors 150G each (Win XP pro, Win7pro, Win7 pro)
GFX- (2) EVGA GTX260 in SLI
AUD- Creative "Sound Blaster" X-Fi titanium pro (needed for the asio drivers)
PS- Corsair TX-750 750w single 12v rail
and a CoolerMaster HAF932 case...just because it looked cool
I'm also running a ThermalTake V-8 cpu cooler, 3-230mm fans, 3-140mm fans and 2-80mm fans
Here's a pic: