Andrew,
#`1 - I think it depends on what type of footage you work with. Some Hi-Def editing (rendering) seems to utilize quad-core systems quite nicely. Standard Def projects don't gain much (if any) over dual-core systems. Of course we hope the situation improves with the next release of Studio, but that's merely a hope that is not based on any statements by Pinnacle.
#2 - Beyond my price point, and it's been too long since I've shopped. Again I think any performance improvements will be dependent on what type of footage you are rendering and what project settings you use. I suspect that in many cases the difference between a $200 card and an $400 card will actually be minimal for Studio users. If I'm wrong, I trust that someone will correct me, and hopefully someone can clarify when additional GPU power will and will not benefit Studio users.
#3 - Yes, the Raptor is simply a higher-performance disk drive, and using it as the "C:" drive would be the best place to put it (for a normal "Studio" system). Old drives used to spin at 5600 RPM. Most now spin at 7200 RPM. The Raptor spins at 10,000 RPM. It's not the only factor, but faster rotation helps you access data faster. They are great drives for the C: drive because your system will boot faster and applications will launch faster. The biggest difference is the "seek" time is shorter. When you copy a single, huge file, there are other bottle necks, but when you are randomly accessing lots of little bits of data spread across the disk, something like the Raptor really shines.
However, as I said, rotational speed is not the only factor, and "normal" disk drives are narrowing the performance gap, and they provide more disk space for your money. I am extremely happy with my 500 MB Samsung drives, and they have had very good reviews at AnandTech.com and good user feedback at Newegg.com. They are quiet, well priced, and the performance is quite good too. The AnandTech article provides benchmark numbers so you can easily compare it against the Raptor - both "synthetic" and "real world" benchmarks.
150 GB Raptor = $170 = $1.13/GB
500 GB Samsung = $109 = $0.22/GB
It's up to you if the incremental performance is worth it.
Happy shopping. 