First off, the 22" cinema display is awesome. . sure it aint the newest/greatest anymore, but in it's day everbody drooled over how awesome they were (and they still get nice drool today)...
Now, the thing is, if you have an ADC radeon 9800 (which is the only one that works in the G5, I think) -- it's got a blue PCB, (and ADC, of course!) then you can run the cinema display without a hitch.
Otherwise, you'd need a DVI to ADC power adaptor, which costs $99.
Hope ya enjoy your new monitor! If anything, your friends or whoever stops by will get a kick out of it, especially since they tend to look like no other monitor on this planet (except of course, other cinema displays from that time)