That is a little on the low side, its best to keep it about 10-12 MB. A battery pull (with the power ON, wait 30 secs, replace batt) should get some some back, but also if you have a media card make sure all your pics and music files are on that, not the device, or check this FAQ out. How do I free up memory on my device? - BlackBerryFAQ
Also, in the browser, hit menu>options>browser config and check the box next to "Terminate slow running scripts"