Yes, you can upgrade to Leopard. (check the tech specs here: http://www.apple.com/macosx/techspecs/)
You’ll need at least a G4 running at 867MHz or better (you’ve got that, of course) and at least 512MB memory (your system will hold two 2 SDRAM (184-pin), PC3200 – DDR400 modules – I suggest you install the maximum the system will hold…)
Memory is available from a variety of different places online or in person: I have found that “The Upgrade Place” (formerly buymemory.com or McDonald and Associates) consistently has the best prices around. The current (spot) price for this memory (iRAM PC3200 1GB DIMM) is $34.00 (USD) each; generic PC3200 memory is priced at $32.00 each. I’d go with the iRAM for $2.00 more….
DISCLAIMER: I am not in any way associated with The Upgrade Place, nor do I receive any compensation for this mention of their site.