Got 147 MB on a 3GS right after reboot.
I'm running Intelliscreen, Backgrounder, Proswitcher, Smsettings. Mail also running right away because I've got Exchange sync.
Don't sweat your free memory unless it gets really low, say under 30 MB. Heck I wouldn't even bother looking at what the free RAM is unless you are diagnosing a problem. The iPhone will keep Safari, iPod, and other apps running (after you first open them) and automatically close them if the memory runs low. If you have multiple tabs open in Safari, that will also be a factor.
Basically, if you have a 3GS, you don't even need to think about RAM. To monitor whether you are going overboard with jailbreak apps that run in the background, just decide if you are getting too much lag or instability.