Just about the only option is the ATi Radeon 7000 PCI, which can be had for about $130. It has analog, RCA and DVI outputs, so a DVI to ADC converter would be needed if your Apple display is ADC. I have one in my G4/Yikes! machine, and it runs great.
I don't think there are any other video card options for a PCI machine.
By power button I'm assuming you mean power button functionality to sleep/power up/power down the CPU itself -- and the answer is, sadly, no. But for turning the monitor on and off it works just fine.
I don't think USB functionality will work either, although I know a few of the DVI/ADC converters have USb connections to the actual computer, so the USB ports may still retain functionality.
It's a little steep in price, but it works perfectly and retains USB funtionality. Don't know about the power button issue -- may have to talk to Apple about that.