Dell are not usually keen to have people upgrade their PCs so if your PC has "on board" graphics it may not have a slot for a graphics card.
Dell often get special motherboards made without slots for graphics cards so check if yours has one before going out and buying a card.
In fact this review here seems to confirm it has no slot for a graphics card
http://www.pcpro.co.uk/buyer/reviews/83738/del l-dimension-3100.html
The same review also indicates it only has 2 slots for the RAM memeory, and both are filled (with 2 x 256Mb).
Before you start upgrading this PC it may be better to start again with a new one, but do NOT get it from Dell but get it from a local compnay who can build it to order for you.