As the folks at Crucial state on their website, only a handful of companies actually manufacture the chips and the boards, and this is not one of them.
It will probably work fine, but you may run into a few hiccups, and reliability issues in a few years. In the worst case, kernel panics, etc.
That's less than $125 per stick, which is basically generic memory. IMHO, a high-end system like the G5 is no place for that kind of memory, it is too integral to the entire system to go cheap.
If you really need it, I would do more research on the company, and what other people think.