Best hard drive setup? (workstation)

My computer is a DCC = Digital Content Creation (workstation)

Got my computer all built, some of you may remember for a week or two back I asked if it was possible to do it for about $600. Here is athreadabout it all (pictures too!)
I didn't get another hard drive because I didnt want to waste money on space I wouldn't use. Problem is, my current 160gb HDD only has 57gb left and its shrinking fast. (plus its kinda slow, avg speed is 40mb/s)
If I had, say, $150 dollars to spend on storage; what would be the fastest/most space/best value way to do so? (ie, get "blah blah" and use current hdd for backup)
I have heard SAS drives are great (mobo doesnt have scsi though), but raid 0 might be better, should I have a dedicated scratch disk? I dont know--which is why Im asking. I plan on doing something come april or end of march at the latest; so if something new/faster is coming out soon please tell.

Thanks for the help in advance.

I posted twice because I didn't see this dedicated hard disk area until after I posted in general storage.