It depends on what interface your displays use (though of course you can get an adaptor for anything).
Other World Computing is currently selling four new dual-display capable cards, all of which are ATI products: the Radeon 9000, 9200, 9600 and 9800. I've summarized below.
There are a couple versions of the
9600 - you'll need the 256MB version because the cheaper 64MB version is a modded G5 OEM card whose ADC port will not work in a G4 without some additional work (I have one and it's great but dual displays is a no-go since only the DVI port gets power). The 256MB version has two DVI ports.
The
9800 256MB is a bit overkill but it is the best card you can get (or ever will get) for the G4. It has DVI, VGA and S-Video.
The
9000 64MB has one DVI and one of the older ADC ports. You'll need an adaptor for the ADC port unless you have an ADC display.
The
9200 128MB is not an AGP card - it is a PCI card and as a result it can be installed without removing your original video card card. It has VGA, DVI and S-Video. You can thus run three displays + a TV if you leave your original card connected as well. It is slower than the other cards but you'll only notice this in games.
I also noticed that they are now selling pulled
GeForce4Ti 128MB cards. These were top-of-the-line (and very expensive) BTO cards from Apple when the Mirrored Drive Door G4s were new. However today they are a bit out of date - the Radeon 9600 will out-perform it in every way, though it is fmore capable than a Radeon 9000 or 9200. It has ADC and DVI.
Personally I'd recommend the Radeon 9600 256MB. It is a tad expensive but it has a lot of VRAM, two DVI ports and it fully supports all the features of OS X's Core Image (the 9000, 9200, and GeForce4Ti do not fully support this).
If the 9600 is too expensive I'd go with the 9000 or maybe the GeForce4Ti (it is an older GPU but does have 128MB of VRAM)
You could always go with an older card on the used market - The Radeon 7000, 7500 and 8500 were dual-display capable, and are cheap off ebay.
give them a try 
