Well the first thing I need to know is what bus the card is operating on. Most 5200's by dell were AGP, and I know they made the 5200 in PCI but I am not sure.
You have several ways to tell what bus the card uses.
1. You can use system info/everest or something similar to see.
2. You can just physically take the card out and look. Google AGP and PCI. They don't look similar so you should be able to tell.
Now after taking the card out, it may be worth it to look at the back of the unit. If you see another VGA port, that will be the onboard. I haven't worked with these generation dells in a long time so I do not remember off the top of my head. If that is present, simply plug the monitor in and see what happens. You should have video automatically, if so, since this is a spare machine it should work ok.
If you do not have another VGA port after you take out the card, you will need to buy a replacement.
Both of the formats are long dead, so the prices are pretty cheap. You can get replacements for the 5200 in AGP or PCI
You can also shop around for used/cheaper solutions if you so desire. You are not limited to the 5200 specifically, anything from that generation ATI 9xxx series should work fine.