All the graphics cards supplied with Macs are capable of supporting two monitors. Most have a DVI port and an ADC port. If you want two DVI ports, you'll need an adapter to convert the ADC port to DVI ($29 from the apple store). If you want two VGA ports, you'll need an ADC to VGA adapter ($24) and the DVI to VGA adapter that should come supplied with the Mac.

You can use a PCI graphics card in addition to the AGP card, Apple do a Radeon 9200 PCI for an extortionate $129.