how can i upgrade my imac g5 power pc running 10.5.8 os to snow leopard

A G5 can't be upgraded past 10.5.8.

(100182)

Read this tip, if you get a machine you can upgrade to 10.6 or later:

https://discussions.apple.com/docs/DOC-2295

Only 2006 and later Macs with an Intel CPU can upgrade to 10.6 or later.

If you decide, at some point, to purchase a newer, used Mac, get something that is year 2007 or greater.

A 2007 iMac, for example, can run OS X 10.6 all the way to OS X 10.9.

The 2007 model Macs and up can take more RAM than 2006 models.