sorry I know you asked this ages ago .. somehow the post was missed
Did you sort the problem? LimeWire 5 will run fine on Leopard if you have most recent java updates (java 1.6 (java 6) with both 32 and 64 bit java).
Download LimeWire 5.5.10 - Lime Wire
The G5 with ibm chips confuses me though. Overall the machine is not 64 bit architecture though is it.