Q: Can I use a flat screen monitor with my PowerMac 7300/200?

Yes,

As long as the flat screen monitor and Mac has the standard VGA connection.

It might also depend on the resolutions and color depth supported by the flatscreen.

Quoted from the PowerMac 7300 page on everymac.com:

"With the default 2 MB of VRAM, this model supports a single display at 512x384, 640x480, 800x600 or 832x624 at 24-bit, 1024x768 or 1152x870 at 16-bit, and 1280x1024 at 8-bit.

With the maximum 4 MB of VRAM installed, it supports 512x384, 640x480, 800x600, 832x624, 1024x768, or 1152x870 at 24-bit and 1280x1024 at 16-bit."

Full page:

http://www.everymac.com/systems/apple/powermac/specs/powermac_7300_200.html

Any monitor with VGA will work. You will need a DB-15 to VGA connector adaptor if your old monitor didn't already use one.

Otherwise you will have find and buy a compatible PCI video card.