An update:

I went ahead and upgraded to 4Gigs of memory and the machine now runs three sessions very smoothly. I was clearly swapping in virtual memory before. I'd also tried stripping mods out on the second and third sessions, as well as turning their graphics down to minimums, but it simply took more memory to fix 'er up.

Unfortunately, my system is one of those that only sees 2.75Gigs regardless of tweaks, so I'm considering taking the 64-bit plunge.

Cheers.