One of the things that Photoshop is widely known for is creating one picture from multiple images, whether it is a panorama or a multi-layered image. Everyone who uses this program recognizes that it demands a lot from your computer. You should consider increasing your memory if you want to boost your speed but don't want to run the 64-bit version. I do recommend that with the 32-bit version you should think about running with no less that 4 gig's of RAM. Most HDR, panoramic or multi-layer images run smoother given the extra increase in memory. Additionally, the additional memory increases Photoshop's ability to track your history of changes.
If you go the 64-bit route one other area that you need to take note of is that your plugins will have to be 64-bit as well. You cannot mix 32-bit and 64-bit processes. Moreover, you might also need to purchase Vista as XP will not be supported.
Basically you need to know that Photoshop CS4 has brought about some very interesting upgrades from the CS3 version, however, if you are considering purchasing the 64-bit version, make sure you are dealing with many gigapixels and/or with extremely complex High-Res images.