It appears that the VM Buffer plugin sort of works like a RAM-based scratch disk so maybe that would improve things at the expense of available memory. Not that this doesn't change the 3.5 GB memory limit that Photoshop has for active use in versiond prior to CS5. I don't have any direct measurements, but with CS4 I've always set other drives as scratch disks prioritized over the boot drive for best performance. Large files can use a lot of memory no matter how much you have, so you very likely will still use the scratch disk.

You can see how efficient Photoshop is running regarding memory and scratch disk by enabling the "Efficiency" indicator on the Info Palette. As the link below shows, the closer to 100% you consistently are the better. If you're near 100% most of the time, then more memory may not do you much good.
http://macperformanceguide.com/OptimizingPhotoshop-Intro.html#ViewingStatus


Best case may be to upgrade to CS5 and upgrade the memory so Photoshop can actively use it all and in a perfect scenario you'd added a second drive as well. Cool