Upgrade from os x 10.2 on imac G4

[Halibut Man]

I have an imac Power PC G4 with a 1.25 GHz processor and 256 MB of Memory currently running on OS X 10.2.8 which was pre-installed when I purchased the machine from new in 2003. I have 59 GB remaining of hard drive ( capacity 74 GB ).
I want to update my operating system - ideally to Leopard 10.5.
Can this be installed and run on my system as it currently stands ?
If not what do I need to do to enable it to do so ? http://images.macrumors.com/vb/images/smilies/confused.gif

 

[r.j.s]

You'd need more RAM for Leopard, but Tiger (10.4) should do ok.

 

[Halibut Man]

Thanks for this rjs.
Reading past forum contributions it would appear that to run Leopard I would require a minimum of 1 Gb of RAM. It doesn't look likely that I can achieve this as I don't want to tackle replacing the RAM in the factory slot myself and I live in the sticks - this leaves only the user slot to fill which can only take up to 512 Mb RAM.
This is the one recommended by Crucial :

512MB, 200-pin SODIMM, DDR PC2700 memory module
CT650014
Module Size: 512MB
Package: 200-pin SODIMM
Feature: DDR PC2700
Specs: DDR PC2700 • CL=2.5 • Unbuffered • NON-ECC • DDR333 • 2.5V •

My question here ( if the above is all correct ? ) is can I fit and use more RAM in the user slot than is fitted in the factory slot ?
Assuming that I can this would give me a total of 768 Mb of RAM - what is the most up to date version of OSX ( which would run smoothly ) that I could install ?