I'd recommend splitting boot and My Documents (Microsoft now (for now) refers to My Documents as "User Files" to D:. While unlike XP there is no more right-click/properties option to move the My Documents folder it is still entirely possible.
I'd recommend C:\ RAID 0/1 with SSDs and then migrate your RAID 5 as your D:. This has an added benefit of not having to worry about losing your files if you get some super-nasty virus that forces you to wipe your C:. For me 512GB SSDs would have been fine. Here is my setup...
C:\ 750GB SSD, single drive
D:\ 512GB SSD RAID 1, My Documents to prevent losing data if I need to wipe C:\ clean.
E:\ 1.5TB Mechanical RAID 1, old mass-storage.
F:\ 4.0TB Mechanical RAID 1, newer mass-storage.
The process to move the My Documents folder to another drive takes about five minutes for me (experienced) and probably twenty minutes for those trying it for the first time.
Create a second Windows user that will not be using the D:\ for My Documents. You need this in case you screw up the user.
Create a third user, log in to all three users at least once (so the settings are applied). clone settings from one user (that is not logged in) to the other user that is not logged in. Switch and repeat and all three users will have the same settings. If you've got other users then obviously don't override their stuff. You'll need to unhide protected operating system files (Windows Explorer-->Tools-->Folder Options-->View Tab).
Log in to the user where the D:\ will become the My Documents folder.
Get a copy of Registry Toolkit which will automate the registry process a bit. There are three find and replace operations you will need to complete and there will be some left-overs that you don't have to worry about. The only thing you DO need to concern yourself with is if you delete the user in Windows then Windows will automatically start to delete your files in that location in spite of the fact that you told it not to (use Recuva, by the same people who made Crap Cleaner if this happens). In that case either open the task manager and kill the process sucking up CPU or preemptively disconnect the drive, boot to the other user and delete the failed attempt if you failed to get this working.
The first find and replace you need to find C:\Users\USERNAME with the hard drive included as there are good reasons we're not finding just Users\USERNAME. Open Registry Toolkit, click on the icon at the top-left, in search for paste the correct path for C:\Users\USERNAME, replace it with D:\My Documents, do not check ANY settings below/left, check ALL Serach/Replace checkboxes below/right, in the next tab apply to all data types and finally in the Replace tab set it to No Prompts. I recommend calling the D:\ folder My Documents as you can change the Windows Places (I use Desktop, My Documents, My Computer, E:\ and F:\ for my places and they're all useful in those open/save dialog windows that uses them).
Pay very close attention to this part so you don't have to trash the user and start over! Now you need to open the registry (Start-->Run-->regedit or create a shortcut to regedit on the desktop). If it is the full path including the C:\ then change C:\Users\USERNAME to D:\My Documents. If you only see a partial match Users\USERNAME then you need to immediately stop and look for the home drive and change it separately (once or twice). If you do not have permission to change a value then it is something from the Windows Update installers, you can safely ignore these; just scroll down to the bottom of the folder on the left/navigation panel and make sure you resume (F3/find next) from there.
Log off and log back on (presuming D:\My Documents exists to other users even if it is not assigned as My Documents to them). If you did it correctly you should immediately see the ten thousand icons on your desktop and all your Windows user preferences (AppData) will carry over.
I use a spare 128GB SSD to test stuff out but also to have a clean clone of Windows 7 in case the copy on my 750GB drive dies for some reason. From scratch it takes me about three days to recover to about 95% of my typically usage and downtime is extremely expensive for me. The fact that Microsoft removed this option (not sure if it's in Vista but it is available in Windows XP) is one of the worst decisions they made.