Since you have a pretty high-end workstation, an SSD would be warranted. Even the fastest computer with an HDD can still be limited in many ways because of the disk latencies.
So if you want a new system drive, Intel X25-M 80GB might be your best bet regarding performance. Since you have an Intel chipset, you may also use two of them in RAID0 and enable 'write caching' for hardcore performance. But, this comes at a risk, though you can use the larger disks for backup if that's required.
I guess it depends on your expectations what product would be best. If you need alot of capacity too, the Velociraptors are decent second choices. But they sure are alot slower than SSDs whenever random I/O is concerned. Go for the Intel SSD if you can; you should be able to use it in your current and your next-generation PC.
Ssd, yes. Or take a second drive of the same model as the first one and build a Raid-0 of them. Though...
- Not usable for backup, and needs a new installation
- It probably won't speed up booting. But loading videos, uncompressed pictures, games, yes.
Software Raid as proposed by Windows with dynamic volumes has serious drawbacks, but it would allow you to keep some volumes untouched, like your documents and the old Win installation. Untouched, as long as everything goes well...
If you use two drives independently, you may put the paging file on the disks that doesn't carry the OS. In case you overflow your 6GB...