Some people will steer you away from spinning hard drives, recommending SSD drives because they are faster, but I recommend you just get a few 3 Terrabyte drives and distribute your video footage so that no one drive is accessing two files at once. You fill up drives fast when making your own 3D video. I personally have not found SSD drives to be big enough to be very useful to me.
TrickMcKaha- While I agree with your other suggestions I have to say the above is not at all practical and in fact just completely wrong from an editing efficiency as well as performance.
It is nearly impossible to manage your footage so that your A-B roll will always put A on one hard drive and B on a second hard Drive. In addition, once you have your clips placed that way then you still can't to an A-B-B-A edit with dissolves anyway. The process of managing clips to always have subsequent content in an edit always on different hard drives will get highly restrictive so that your story is edited based on hardware management for performance rather than content. You'll be spending all your time making sub clips from master clips that you can place.
The performance on data throughput of an SSD is roughly 4-5 times the data throughput of the fastest HD so even if you do manage to play the musical HD game it still will be twice to 2.5 times slower than the single SSD. Of course you could always spend the money for multiple HD in raid config but this could require 4-5 drives in RAID0 to achieve that. Hardly practical considering the cost.
The argument that you "need" 3-4 TB of disk space for most projects is just not true. My longest projects that end up being 2 hours long in finished editing that hold 15 hours of raw video clips, music and graphics occupy at the most 120Gb of space on my work drive, a 256Gb SSD Vertex 4. It is rare that I build huge projects like that and my average project folder size is usually 50Gb in size for all the files.
What you should do is plan a second 128 Gb SSD to take care of your temp tiles for rendering, or even use a HD for this purpose. There is little gained from using an SSD for temp files during the rendering but it does help. The main advantage of the SSD is for your source video to be able to achieve adequate clip feed rate to play the timeline in real time. This allows you to see the story as it will p[lay and make quicker decisions on how to edit your story. But you still need a fast CPU and GPU as well.
The 3D editing process with Sony Vegas depends on very fast processors and graphics processor. I use an i7-950 here with 12 Gb of ram which seems adequate. I also use an Ati Firepro V8800.
Basically, I can view two full streams of MVC video at 24 fps 1080 x 1920 on my 3D vizio with dissolves with hardly a glitch at the dissolves. 1080 60p is only achieved with single clips.
If you really want speed, then the latest OCZ dual channel SSD card with built in dual controller is the way to go but these are expensive, about as expensive as the high end Firepro card.
I also have two 3Tb HDD in my system but these are for archiving finished projects and library archives I may use in other projects. I would always make a copy of that footage onto my work drive for editing so it is accessed at the fastest data rate.
Finally, you can also increase your work efficiency by using an SSD in your C-drive. This speeds up reboots and when rendering speeds up Vegas Pro application access during rendering and other editing functions. I would not have the OS and applications, Project work drives, and temp files on the same SSD. In this final configuration, I have three SSD 256 Gb drives for each of these functions. Each of these drives are the OCZ Vertex 4 series which are the fastest data throughput on the market and only exceeded by their dual channel system which feeds video at double the vertex 4 rate. ( 550 Mbs vs. 1Gbs )
If you want to add a good tool to analyze your bottlenecks during rendering or playback, check out www.Moo0.com
they have some great analytical tools to do this.