I think some of your problem in 32-bit was your video card. Remember that XP-32 bit can only address a total of 4GB of memory. With 1GB on your video card, that only left 3GB for everything else. So it's not just that you went from 4GB to 8GB, but more like 3GB to 8GB. Even just the upgrade to 64-bit would have bumped you up a GB.

This is just something to keep in mind when you're trying to figure out where your system bottleneck is. If someone were just looking at your old spec's they may have assumed that you were not bottlenecked. It looked like you had plenty of RAM to spare (athough you didn't), and your CPU usage was pretty low too. But in truth you were actually bottlenecked by your OS.

As a side note - if you have a 9550, I don't think you can hold 12GB of RAM. All the 775 motherboards I've seen, cap out at 8GB - 4x 2GB.