Do you have specific reasons to upgrade or do you just feel like it
lol
That PC should be doing everything I can think of pretty well.
Raid is simple enough to do, I assume you want to stripe (raid 0) them. Just plug the new on in, set to RAID in bios and then set it up on boot. You'll need to format the drives though.
4GB or RAM is great but are you using 64bit windows and or RAM intensive programs?
Blu ray holds no benefits on that screen and it's a bit pricey. You could output to a HDTV via the 8800GT but to be honest I'd want a more silent PC solution to do that sort of job and hook it up to the TV