My two cents:
Bare metal hypervisors like VMware ESX and Microsoft Hyper-V Server don't provide a local user interface, so as noted, you'll need a second system to access your virtual machines.
A hosted virtual machine solution like VMware Workstation/Server or Virtual PC/Server is a better choice in your case; however, most of your hardware won't be virtualized. If you want to use the Media Center features of Vista Ultimate, run that as your primary OS.
If you want to use the Catweasel under Windows, you'll have to use a 32-bit version of Windows. As far as I know, there are no 64-bit drivers for the Catweasel.
If you're fine with not using the Catweasel and your OEM version of Vista is 64-bit, go for that and don't run Windows XP at all, assuming your video capture device has 64-bit drivers. The XP v. Vista debate is really just that. Ultimately, you need to decide for yourself which version of Windows provides the functionality you need. Stability is largely dependent on hardware, device drivers, and third-party software, particularly antivirus (kernel mode file system filters) and other "security" sofware.
I don't know if you can run Mac OS X under an x86 virtual machine.
If you're serious about using a disk array, don't use your motherboard's onboard RAID controller, which is most likely software-assisted, and don't use Windows' built-in array features. Buy a third-party SATA/SAS caching array controller from Adaptec, Promise, or some other vendor. If you don't care about data loss, create a stripe ("RAID 0"); otherwise, create a mirror (RAID 1). A two disk stripe or mirror will perform about the same for reads. Writes on a mirror are similar in performance to a single disk. Performance ultimately depends on the interconnects and how the array is implemented by the hardware and software.
EDIT: Additional note on caching array controllers: with write caching enabled, writes (from the file system's perspective) happen as quickly as your host can move data to the controller's write cache, taking into account bus contention and cache overflows. Write caching is a great feature, but make sure the controller has a battery backed cache to prevent data loss during a power failure. Couple it with a journaling file system like NTFS or ext3 to prevent data corruption.
