After several months of the site running like a top, it seems the past 2-3 weeks I've been running into processing/memory limits with both servers during the daytime US hours. The site is nearly inaccessible from 11am-2pm pacific due to the traffic loads.
I've done a bunch of small tweaks without getting much added capacity, so I've gone ahead and ordered up some new servers from my host. It'll take about a week to get everything moved over and running on the new hardware, but I should have about 3-5x the computing power available to the site when we're all done.
I'm going from two dual xenon processor boxes with 2Gb of RAM to the latest dual quad-core xenon intel chips for 8 processors per box instead of 2, and with 8Gb of RAM. The database should be kicking ass on the new hardware and hopefully the apache/coldfusion web server can scale up nicely as well.