This is a "seat-o-the-pants" response, but is based on many responses in this forum. It seems that PrE 9 (though still a 32-bit app.) works better for most users, regardless of OS, than did PrE 8. Someone suggested that PrE 8 was the "Vista" of PrE. I cannot confirm, but the responses do seem to lean that way.
Now, be a bit careful, as many Projects will Open and edit fine in a newer version of PrE (and also PrPro), but that is not always the case. There can be issues. Most success will likely be related to the Effects, Transitions, Themes, etc., used in the older Project, but there can be other issues too. I always recommend that Projects started in previous versions be finished in that version - that is always the safe route.
Wish that I had concrete recs. for you, and others might be able to give you reports "from the trenches."
Further Bill's suggestion, I recommend you download the free trial of version 9 and give it a test drive. That will tell you how well the program performs with your particular computer specs.
I've just recently started using Windows Pro 64-bit and PE 8 and I will wait a bit to use my new Premiere Pro CS5 suite. I've ordered the book Hunt recommended for CS5 from amazon.
However, I was thinking also about buying and dowloading PE 9 to do my next video. I've got a German song to do for the advent calendar of the German ukulele club. The pressure is on! Should I get 9 or stick with 8? I'm feeling like living dangerously lately since getting the CS-5 suite and having a few more bucks on hand! hehehe