Its a good idea if thats an intel iMac. The G5 has a mini-vga out vs. the intel mini-dvi.
For an intel iMac:
Mini-DVI to DVI adapter= $20
DVI to ADC adapter= $100
Total= $120
For an iMac G5
Mini Vga to VGA adapter= $20
VGA to DVI adapter= Between $200 and $300 (I think)
DVI to ADC adaper= $100
Total= $320-$420
edit:
ok, nevermind. Last time I checked you had to buy this whole expensive box and everything. Now all you need is http://sewelldirect.com/dviaftovgam.asp
I think thats what you need if you have a G5
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...