Hi Mike and Carlos,
Just a few remarks. Whatever operating system you decide to install, it should be a 64-bit operating system. Any 32-bit OS is now obsolete and will impose severe limitations, especially with PixInsight and other memory and processor intensive applications.
Regarding memory, I would seriously consider installing 8 GB of RAM at least. This is more important than the processor.
I fully agree with Carlos' suggestion regarding video hardware. Who knows where the market will go in a couple years. CUDA is nice but it is not a portable solution. OpenCL is the way to go and definitely where PixInsight will go during the next years, but this is just a project and PixInsight has many more priorities in the short and medium term.
If you want to try linux, since today's HDD are so large, just keep 100 or 200 Gb of unused space on the disk (a new partition, not formated)
Much better and cleaner if you install Linux on an independent hard disk. Install Windoze on the first physical disk unit and then Linux on the second disk. The Linux installer will install a boot manager that will allow you to choose which OS to boot from.
Unless you have a strong reason to use Linux, I recommend PC-BSD (a desktop-oriented version of FreeBSD) instead. Faster, more efficient and currently the best platform for PixInsight.
Also keep in mind that Linux support for nVidia cards is very problematic these days (search the web for "Nouveau Linux" and you'll know why), and in practice it is impossible to get (stable) hardware graphics acceleration under Linux if you have an nVidia graphics card. This is due to the extreme difficulty to remove the Nouveau driver that all major Linux distros install by default. FreeBSD has none of these absurd problems and installs the proprietary nVidia driver, which provides full OpenGL acceleration.
If you want stability and keep things easy, go for Ubuntu on Gnome. If you want to be on top of the wave, Fedora and KDE
Here I disagree. Fedora 14 with KDE 4.5 is very stable and IMO better for serious computing purposes. However as I've said PC-BSD (which also provides KDE as its default desktop environment) is better than Linux, IMO, especially if you want to work with PixInsight.
FWIW, the primary development platform for PixInsight (that is, the platform where PixInsight is being actively developed and tested before porting it to other platforms) is right now Fedora 13 with KDE.
A bit off topic but .... I intend to use two disks in RAID1 configuration - would I need to partition the second one similarly?
When you use a RAID configuration, you actually have no control on the way each individual disk in the array is partitioned. You (and your operating system, at a high level) see the entire array as a single logical disk. Normally after defining the RAID array, either using a hardware RAID controller or a software based RAID implementation, you partition it as if it were a single disk drive.
With a RAID1 configuration you have two or more disks mirrored with the same contents. It is not the best RAID configuration in terms of performance (although its read performance is very good), but it is the most secure. I use a RAID1 array of two server-class 750 GB Western Digital disks in my main UNIX workstation.
A last advice regarding RAID. Beware of "hardware" RAID support provided by many motherboards. In most cases they are really software RAID implementations using cheap dedicated hardware on the motherboard and a proprietary closed design that can cause a lot of problems. With modern processors, pure software RAID implementations are more efficient and you don't depend on a closed disk management system.