Slow slow and slow

I'm running FCP X and things (rendering) is just unbearably slow.

I turned off background editing because it's just too difficult, and render manually as needed.

I am doing a lot graphic stuff (moving and scaling images, green screen, cropping, etc.), which I know requires some power.

So here's the thing: I'm thinking of just getting a new computer, but only if that's going to make a big difference.

Question is, what should I get that's going to really be speedy.

Here's what I have:

Apple Mac Pro

2 x 3 GHz Dual-Core Intel Xeon

6GB 667 MHz-DDR2 FB-DIMM

OS 10.7.5

I have media on external drives (7200rpm)

WHAT SHOULD I HAVE/GET?

Thanks.