I have old Nvidia GeForce 8400 GS. I dont think they are power hungry.
I dont know if I will ever add any other component that will consume significant amount of power, however I just wanted to leave some room just incase if I need one later.
The 8400 GS takes almost nothing. Even if you seriously overclocked your CPU you could still get away with a 300 W PSU. Something around 500 watts bronze certified should last you another 6 years, like this: Antec EarthWatts EA-500D Green 500W for $73.
That will let you upgrade to a light gaming card or any combination of components you might decide on. It is also energy efficient which will reduce operating cost.
Regarding the harddrive, I chose the other one because it's transfer rate was 6GB/s instead of 3GB/s. I stream tons of samples from harddrive, so I thought faster one would give me some advantage.
Also, I had thought about getting a SSD but, I am not sure what I can do with anything less than 500 GB, unless I use that just for OS. The virtual instrument plugins that I run in the studio are about 50 GB each, and I run about 5-6 of them.
So install OS in one drive and install all other Applications in a separate drive? I was thinking of creating multiple partitions (2-3) for that.
I will be using Sonar X1 (Producer)and Fruity loops 9(Producer). I may use huge VST Intrument plugins like Synthogy Ivory, Trillian Bass, Superior Drummer, Miroslav Philaharmonic Orchestra, Omnisphere, etc at the same time. Although Fruity Loops' "4 GB RAM limit" may limit me.
Audio interface will probably be RME fireface 400. I may add my current Echo Mia (PCI card)as well.
It is a good idea to have the OS and audio on separate drives. Also, the swap file should be on a different physical drive from the audio as well. It isn't that the applications need to be on a separate drive, but the audio itself including the VST data files (which is basically audio samples, gigs of them.)
For compatibility reasons it is recommended that you add a firewire card to the machine. Texas Instruments firewire chipsets are more reliable for audio use (unless you want to spend time tweaking the firewire settings, which the RME drivers do allow) and so you might want to add a TI-based firewire card to the build.
FL Studio also supports 64-bit, and you should install 64-bit windows. I dont know if the bridge runs the VSTs as a separate project (like in REAPER) but its still advisable.