[fryke]

Generally, Mac OS X will always try to maximise the use of all the RAM you've got, so "top" showing you almost no free RAM is not necessarily a hint that you need more physical RAM. What _is_ a good hint is if the computer gets very slow when doing various things at the same time and you hear constant disk thrashing, i.e. when virtual memory writing and reading is a constant problem.

More RAM will _certainly_ give you a performance increase. If you can afford it now and it seems rather inexpensive, there's almost no reason _not_ to upgrade. (I'd look whether two 512 MB DIMMs are installed currently, because replacing one with a GB DIMM would only upgrade it to 1.5 GB, of course.) However, I'd personally say that 1 GB on a PPC Mac running Tiger is enough for iLife use. It _is_ important to have lots of disk space free for projects in iMovie and iDVD.