Your hardware is within acceptable specifications for blu ray playback. Which leads us to software issues.
I gave up on PowerDVD some time ago and use Arcsoft Total MediaTheater for blu ray. You might want to follow Dean's earlier advice and download their free trial and see if playback improves:
There are some free players that can handle the main movies in blu ray, but you lose the menus, and it is much more difficult to access extras. But if you want to try one, the best is Media Player Classic HomeCinema:
There may be a setting in PowerDVD to enable hardware acceleration from your video card. Check to see that it is actually enabled.
Edit: By the way, don't know if you'd be interested in buying that Arcsoft player, but its regular price is $99 and you can use this coupon code NYARC30 to get a 30% discount. Need to test it extensively first.