can i use any monitor with my g5

It depends on what the monitor actually supports, the size of the monitor, & the Graphic card capabilities.

At the Apple Icon at top left>About this Mac.

Then click on More Info>Hardware>Graphics/Displays and report like this...

NVIDIA GeForce 7800GT:

Chipset Model: GeForce 7800GT

Type: Display

Bus: PCI

Slot: SLOT-1

VRAM (Total): 256 MB

Vendor: nVIDIA (0x10de)

Device ID: 0x0092

Revision ID: 0x00a1

ROM Revision: 2152.2

Displays:

VGA Display:

Resolution: 1920 x 1080 @ 60 Hz

Depth: 32-bit Color

Core Image: Supported

Main Display: Yes

Mirror: Off

Online: Yes

Quartz Extreme: Supported

Display:

Status: No display connected

Power Mac G5: Display Compatibility

http://support.apple.com/kb/HT2707?viewlocale=en_US

http://support.apple.com/kb/sp96

You may use any DVI equipped monitor that has a maximum resolution as supported by your graphics card.

At a minimum, all G5 graphics cards will support 1920x1200.

Cards equipped with a dual link DVI prot will support resolutions above that.