Ok i have a pentium 4 dell dimension 4600 desktop, it came with 512mb ram and a nvidia geforce fx 5200 graphics card. I recently got 1 gb of ram put that in my computer everything was fine then i got the new XFX geforce 7800graphics card and put it in the agp slot and it fit in fine then i connected it to the power supply through these wires that were also connected to the power supply and then i screwed the card in and put the side of the tower back on. so i reconnected everyhting and when i booted up my computer it showed that the monitor was on but nothing appeared on the screen and it just stays black. so then i put my old card back and it worked fine and when i put the driver disc for the new video card a message pops up and says go to www.nvidia.com and download the VGA driver and i dont know what that is. so can someone please tell me what to do so i can fix this problem. if you need more info let me know i would really appreciate any help.

I'm guessing you didn't uninstall the drivers for your old card before you put in your new card.
When installing a new Video card this is what you should do.

1. Uninstall Video drivers for old card.
2. Shutdown computer.
3. Remove old video card.
4. Install new video card.
5. Start compute up.
6. Install latest drivers for the new card

(taken from Jimbo420)

Also while you have the case open tell us the make and wattage of your psu,
and the amps on it's 12v rail(on the sticker), just so we can make sure it has enough power to run the graphica card.
btw welcome to Techspot

thank you, i did not uninstall the older drivers, but do you still think that it would cause my monitor to go blank when i put in the new card?

Yup, that'll do it.

Your going to need a new PSU aswell. What PSU do you have currently. The extra RAM and the nVidia is gonna suck some juice.