It has been in my experience with iRealQuickSMS that it consumes a lot of memory. The concept is great but I just don't think my 3G phone is up to the task most of the time.
You can try going to sbsettings click More>MobileSustrate>and disabling iRealQuickSMS. Look and see how much more memory you gain. (Note: this is used to debug memory and troubleshoot compatibility issues.)
With WB cleaned out mostly, and iRealQuick disabled I'm cruising at around 47 MB free memory. Much, much better experience.