Except that if it's Ram-starved under OS9 then it's got so little Ram that OS X will probably not even run
The best thing is probably to upgrade your Ram to 384 or 512 MB and run either Panther or Tiger. OS 9 is OK, but tends to lock up quite a bit (or at least in my experience) and there is the issue of browsers as already mentioned. X should perform reasonably well as long as you have a sizeable amount of Ram, although of course with the iMac you are limited by the built-in video card.