Also, if you do go to 10.2/10.3, you will need to double-check your firmware before you install it. IIRC, if you have an old slot-load iMac, if you don't do the firmware upgrade first you can cause the screen to be unresponsive (can be fixed, but not easily without another Mac on hand!). The 333Mhz iMac is a tray-load, so not sure if that applies, but still make sure you have the latest firmware installed.
BTW, you can probably find lots of good info on the capabilities of your old Mac on lowendmac.com, where they list specs and stuff like maximum RAM etc.
My parents have my old 333Mhz iMac, running 10.3 (can't remember how much RAM). Slow, but usable for simple email, net and Word things.