How can I get the best performance out of my Mac Pro?

I am currently running an 8 core Mac Pro from early 2008
I am looking for the best possible way to set up my computer.

Here is what is inside the box:

18gb RAM

ATI Radeon HD 5870 graphics card (all 3 display ports are being used)

1.5 Terabyte 7200 RPM Seagate Drive
1 Terabyte 7200 RPM Western Digital Drive
2X 320 GB 7200 RPM Western Digital Drive

I am currently running Lion Server


I use the computer mainly for video editing and compositing with some 3D rendering. I am very happy with it but I feel like it could be much snappier and with the hardware installed that it should be performing much better than it is. I am going to back it up and re-install my OS but I was wondering if I should go back to Snow Leapord Server, as it was the operating system that came with my machine. I am thinking about setting up my two 320 GB drives as a striped RAID 0 and running my OS and heavier applications off of this. Would there be a noticable performance increase by doing this?
Is there anything else I should be doing to make it faster?

I am toying with the idea of buying a small Solid State drive to run my operating system off of but I would like to avoid spending any more money at this point.

Thanks for your help!