Thanks for the welcome, Redrobes.
Okay, my file specs were definitely off a bit. The image is 20122x11634 @ 450 px/in. Apparently the file size was way off too, I must have been thinking about the old version of the file. The PSB file is about 3.7GB saved, and PS consumes around 7.75GB in memory while I'm working with it. So tilt's point is a good one - go with 2-3 times the amount of RAM relative to the saved file size.

Definitely a good explanation of the 64-bit and RAM limitations. A couple of other points I seem to remember from when I was researching prior to transitioning from 32-bit to 63-bit Vista: because of how the pointers work the most RAM any given application can use on a 4GB 32-bit system will probably be in the neighborhood of 3.2 to 3.7GB. Most 32-bit OS will only actually detect 3.7 to 3.8GB of available RAM, depending on your setup. Sometimes the type and configuration of your video card will also affect this, as some setups allow the video card to use a portion of system RAM as a sort of virtual VRAM. So, bottom line if you're working with files larger than about 1.5GB, you will probably benefit from upgrading to 64-bit. Just be sure to keep in mind what Redrobes posted - you're entire system has to be 64-bit, the CPU, OS and the application.

For the most part (with the exception of the VRAM and memory pointer issue) the video card has very little impact. CS4 enables GPU support to increase draw speeds on the monitor, but I think that's about it.
However, that may be changing now. The new DX11 cards enable the computer to offload computing tasks to the graphics card. This isn't something CS4 is really capable of taking advantage of, but I've heard CS5 might be able to utilize this feature. I'm not sure how much of a performance boost you would see, though it might not be that great if only the GPU itself is used as PS doesn't really use that much CPU power. It would probably be of greater benefit if this setup allowed PS to utilize the VRAM on the card. Anybody tried anything like to see if it makes a difference? And then there's the issue of cost: upgrading to CS5 aside, the new nvidia DX11 card start at over $400. Upgrading to 64-bit with more RAM is definitely a better option.