a RAID wont help you when it comes to standard SATA drives. In fact, it could hurt when it comes to performance in catalyst. I would strongly suggest getting at least one SSD (even a small one) where you can migrate the files you are using most in your show, you'll see a significant increase in performance.

Also, with the HD4870, remember it only has one DVI port, and then the MiniDisplayport, which you will need adapters to use. I just did a show that had three new machines on it, one with a 4870, and two with Nvidia 8800s, and I couldn't discern a difference between them, and the 8800s have two dual link DVI ports built in.....