Do you have 10.4 available? I put 10.4 on a 7 year old Mac (450MHz G4) and it runs great.
10.4 also adds a lot of neat stuff over 10.3.
I know it's tough when perfectly good equipment becomes obsolete just because someone made a decision not to support it, but that's the way open source is. Since it's all done by volunteers they often don't have the time or equipment to support older versions of, say, operating systems. Apple doesn't support 10.3 anymore, so I think it's fairly safe for an open source project not to support it anymore.
Thanks.
-Phil