However, for HD editing you will see significant performance improvements with the latest version. This performance boost can be attributed to two factors. The first is that Premiere Pro CS5 only runs on a 64-bit platform. (You can read about 64-bit computing here.)
The other factor is Adobe's new Mercury Playback Engine. Basically this technology is able to leverage the full power of display adapters using Nvidia's CUDA technology in tandem with the computer's CPU. Here's a short video from Adobe that explains its value:
Ugly graphics aside, one can see that the great value is the ability to work with compressed HD video formats like AVCHD and edit natively in real time with no rendering. Exporting video is also very impressive.
At the same time, you need to determine whether or not you need all of this horse power. Premiere Elements 8 is an excellent basic editing application that may well suit you needs (and save you hundreds of dollars.) And it doesn't demand the 64 bit computing environment which is now required for Premiere Pro. (You can check out my review of Premiere Elements 8 here.)
Trial versions are available for both products at Adobe.