Yes, assuming you could find a copy of 10.5, then you would be able to upgrade, but that's as far as you'll be able to go with a PPC system. Anything requiring 10.6 or later will require a new computer.
The tricky part will be finding a copy of 10.5, since Apple is prepping to launch 10.8 soon, and virtually all PPC products have hit vintage status, it's unlikely Apple is still selling 10.5, leaving you with more gray market options such as ebay and craigslist. If you look around you will probably still be able to find it, just make sure that it's not some bootleg copy that is a burned disc or something, or the restore media for someone's iMac G5 which went down for the count with bad caps. If it's not a pressed retail disc, you don't want it. Since your system came with 10.4, any 10.5 retail disc will do.
I've got an Imac G5 1.8 GHz with 10.4.11& 2GB ram,
running CS5 etc. I am looking to buy 3D software but they all need 10.5 + Can I upgrade? If not, is there an older
app. that I could get? Where can I find listings of older
Apps that would suite my OS? Cheers

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