|What do you want to be able to do?|
If both monitors having an identical display of what is on the screen in all circumstances is fine, you don't necessarily need a video card.
If you want the two monitors to be able to have different displays, that may cost you more than you thought it might, if you choose a video card with a more recent chipset.
I've looked up your model before.
Dell™ Dimension™ 2400 Series
It's a minitower and has onboard AGP graphics - you have only 3 PCI slots, available, max, and no AGP slot.
If you get a video card, it will disable the onboard graphics when it is installed.
If you want to be able to connect two monitors to the video card, it must have two monitor ports that can be connected to your monitors, either directly or by means of a DVI to VGA adapter at the port (you can't adapt VGA ports to DVI use). Some cards come with no DVI to VGA adapter, some come with one, some come with two.
Your power supply has either a 200 watt or 250 watt capacity - typically minimally sized, as is the case with most brand name systems.
I looked it up - it is not proprietary - it's a standard (PS/2) sized ATX power supply, with standard ATX wiring.
If you buy a cheap PCI video card with an older video chipset you MIGHT be able to get away with not having to up the capacity of the power supply, but if you use a better PCI card with a more recent chipset you WILL have to get a power supply with more capacity, a minumum of 300 watts, or more.
If you use a cheap PCI video card with an older chipset, your ram will perform better, your video will perform a little better, but otherwise the video may be no better or a little worse than what you already have, since the onboard video is AGP.
Any better PCI video card that requires an extra power connection from the power supply will probably require you get a power supply with more capacity, and preferably, one that has the extra wiring and connector that can plug into the card, though sometimes the card comeswith an appropriate wiring adapter.
If you use a better PCI video card with a more recent chipset, your ram will perform better, and your video will perform a lot better.
In any case, you should look on the card manufacturer's web site at the specs for the model and find the minimum system power supply capacity need for a system, and often the minimum current the PS must supply at a voltage, with that card installed on it. That's often listed under system requirements. If you see that, the PS you buy, or the used one you use, must have at least those ratings.
If you DO go for a video card with a more recent chipset and replacing the power supply...
- any standard (PS/2) sized ATX power supply will fit in your case and work with your mboard, but if the video card requires an extra power connector be plugged into it, it's preferable the replacement PS has that.
- most new (and more recent used) PSs come with a main connector that can be used on either a 20 or 24 "pin" socket on the mboard, so you don't necessarily need to find one with only a 20 "pin" main connector.
- the original PS may or may not have a white wire at position 18 on the main connector, and some new ones do not - that is used for an ISA slot / ISA card function - it doesn't matter whether the replacement PS has the white wire or not, since your mboard has no ISA slots.
The PCI slot on the end of the 3 slots closest to the center of the mboard should NOT be used for anything except a PCI video card, because that slot is forced to use the same IRQ as the onboard video, or as the AGP card in the AGP slot if you have one, and that situation frequently causes problems for other than a video card. A PCI video card can probably be installed in any of the 3 PCI slots, but it's NOT a good idea to install other than a video card in the first one.
Most people are not aware that when you connect two monitors to a video card, there are a few commonly used programs that will only produce a display on the monitor Windows sees as the Primary monitor.
E.g. Power DVD will only actually play DVD playback in it's window on the Primary monitor.
If both monitors having an identical display of what is on the screen in all circumstances is fine, you have another alternative.
You can get a video splitter box (hardware in a box) and a video extension cable and connect them to your existing single video output port, and connect your monitors to the box. A video splitter box is the same thing used in stores to display exactly the same display on two or more monitors at the same time. If both your monitors are VGA that can be relatively inexpensive, and in any case you will NOT have to replace your existing power supply. They have a built in amplifier, so if you want one monitor to be more than 6 feet or so from the computer, that's no problem as long as you use a heavy duty video extension cable to the farther monitor as well.
The video splitter boxes here:
(NOTE that the simple video splitter cables will usually yield you POOR results, according to what I've read)
VGA Extension cables: