What does the socket say? Does it say LGA370? Cause thats the socket for PIIIs. You can also look up all this information from dell by typing your product number in their support page. The product number is a sticker on the back of your computer.
Finally, I have to say that idiotec is right. I contacted customer support a few weeks ago and asked them if they modified their mobos to accept only dell ram. They responded by saying that I could use any ram from any compnay as long as it was 184 pin DDR.
P.S. Check to see the maximum ram allowed on your motherboard. Some motherboards only allow 2 sticks of 256, while others will allow 2 sticks of 512. Also, don't overdo things. Getting the 6600GT for your old pIII dell is a waste. FX5900 should do.