Might be pushing your luck with that 450w psu, especially if it's a no-name/off brand or low quality psu that's as old as the rest of the system.
Make sure the cable to the monitor is in the gpu, not the motherboard and make sure the monitor is set to whichever connection you are using.
Is the monitor turning on and giving you a black screen or is the monitor not even switching to On?
Have you reset the cmos/bios by pulling the battery?
Does the prior card using the same connections work?