Ok, so I did a little testing last night. I wanted to compare how much ram a freshly installed non-jailbroken phone has compared to after it is jailbroken.

Phone: 3G 16GB
Firmware: 2.2.1

I had to use a program called MemStatus to use for benchmarking because it was the only memory tool i could find for a non-jailbroken phone.

1) Fresh legit install (used restore in itunes):

2) 1st Benchmark - all I did was open app store and download MemStatus & rebooted:

3) Next I jailbroke the phone, and rebooted one time:

4) Then I ran the MemStatus again:

As you can see there is less free memory available to the phone after jailbreaking. I did not add any contacts, mail, open safari, etc. NOTHING was done on the phone. This is mostly just FYI info if anyone was interested in a comparsion..