Here's the breakdown...
I'm an average computer user, but I'm building a pretty powerful desktop. I mainly use it for multimedia/surfing/moderate gaming (that will change when SC2/D3 come out).
This is what it looks like :
i7 920 /w TRUE hsf
EVGA GTX 295
Gigabyte UD4P x58
Corsair Dominator 1600mhz 8-8-8-24 6gb tri channel
I currently have a Samsung Spinpoint F1 640gb, but I wanted my HD to match the power of my desktop as a whole. Will I see improvements if I go with a Raptor or go RAID 0?
Beware Ssd! Some are pretty quick, but so many are hugely slower than an Hdd! So don't buy any Ssd hoping its name will make it fast.
Your choice over Velociraptor might primarily depend on if you accept their noise. Do you have an opportunity to hear one? And if you really accept noise, consider a 15000/min Scsi disk with an added host.
Raid-0 is very efficient on big files, for instance to display quickly a directory of .raw pictures. For an OS, it is sometimes good, sometimes useless.
Xp home will use 1 of your cores, Xp Pro 2 of them. Hope you have the right OS for your quad-core.