This should be very close to your PC.
2.67-GHz Pentium 4
512MB of DDR333 SDRAM
Hard drive (GB)120GB
Monitor size and model- 19-inch Dell M992 CRT
Graphics- 128MB ATI Radeon 9700 TX or Geforce4 420
Case type- Midsize tower
Open external bays, slots- 1 bay, 2 slots
1X AGP 8X slot
4X PCI slots
The trouble with Dell (most models) is their powersupply. Here is your problem also... You have only a 250Watt Powersupply!!! That means the cards you can run are not very new at all. I would go with an ATi 9800Pro or 9800XT only because ATi cards require a lesser powersupply, and since Dell is dumb and out to make alot of money, they put on a special MOBO that uses a "special" PS input plug socket with a very unique shape. You can only buy a new DELL powersupply, which will cost you about 200% more then buying elsewhere.... Sorry to say, but here is a link to a graphics card you may want to consider.
This is a 9800Pro, for about $260 dollars. Problem again is Powersupply, being that ATi recommends a 300Watt + PS!
Ok, so now that you are stuck lets try something else. Try a 9600XT will serve you fairly well, and only requires a 200Watt minimum
Try this one, or get a new powersupply, or a new PC...
For a good PC, i recommend www.cyberpowersystem.com