I am running CS4 and when I am editing I am still getting choppy footage through my preview monitor. But when I check my task manager under performance my PC is only using 3.40gb of RAM and between 2 and 20% of my CPU ( I have 8gb of RAM, 3.6 ( or 4) quad-core processor and 64bit vista).

Is there a way to tell it to use more so I can play smoother footage?

Thanks!

Even though you have a 64 bit OS, PPro CS4 is a 32 bit app and cannot use the extra RAM you have onboard.
That's why Adobe switched to 64 bit for CS5.

Same as above really. There is no way of telling CS4 to use more RAM, it will use what it needs upto the 32bit limits of the program.

so would this be a good reason to move up to CS5? If so, are there any other major perks that CS5 has to offer?

The Mercury Engine is fantastic in CS5. I just upgraded on my i7 system that was still struggling with Cineform clips in CS4 that had unsharp mask, luma corrections, and color corrections. I couldn't get fluid playback of the timeline on those clips without rendering. Now I can stack multiple layers of those clips and it plays back flawlessly.

I'd like to try CS5 on a Core2 system to see how it behaves, but I don't have one anymore. It definitely is a huge improvement over CS4.

Not only the MPE is terrific but you can also tell Premiere how much ram to use.

so i guess im going to be upgrading then... kind of excited!