Ok, you want a speedier disk/drive access...partition 20 gb on the drives in Mirror formation for reliability, the other 180 in stripe. You'll get a substantial boost in disk access but put your data at risk of loss in a drive corruption, but since you do video (like me a bit with DVD's) this data isn't really critical.
512 mb memory is plenty (get one stick if possible to allow future sticks since many mobos only have two slots)
Remember a couple of things, BTX is around the corner, so is PCI express, you bragging rights will be short lived, maybe x-mas at the most this year. I would make this my xmas present since both of these are not too bad of features to wait for.
Mirroring PARTITIONS that are on ONE drive will do absolutely nothing for performance or reliability. The whole purpose of RAID is spanning data across multiple drives
512MB RAM is good, but 1GB is better.
I already suggested a Dual Layer burner for $88...
BTX is not as close as you think. It is still in the baby stages. ATX will be here for a while to come yet. Even if BTX does come out, my guess is most motherboards will be available in both BTX and ATX.
You are NEVER ahead of the game when it comes to computers. You are ALWAYS going to be putting things off if you wait for the next best thing to come out.