There are no VGA to DVI adapters! You can't do that with an iMac G5, except if you can get hang of a VGA to ADC adapter, but I don't think they excist. Get yourself a nice iMac Core Duo, and you'll be much better off.
BTW, I wouldn't do it, since the old ACD's are crap compared to todays standard. They have a responsetime of 40 ms...