Well, that depends on how much storage you need. If you're content with the 120gb, then might as well go the SSD route, but if you think you'll use more space, even 2x 150 is 2.5x the storage, and 2x 300 is 5x the storage for probably close to the same cost.
To me, 120gig is laughable... I think the smallest HD I've got is 500gig. I don't think I'd go SSD until they're at least 250gig and affordable. I can wait. If you can afford it now, go for it... You can always get a second or third one down the road and stripe 'em for insane throughput.
Then Raid 0, 3 of the 30 gig Vertex drives. That will cap most controllers right now. If you can't take the space hit then 2 of the 60 gigs is just a bit more.