I am not sure which form factor you have, Dell has 3 form factors listed. Small form factor, small desktop, and mini tower are listed.Two of those can only take low-profile/half length cards. Your options are quite limited, sorry to be a naysayer but I think you'll be better off saving for a new system as opposed to getting another monitor. That's my recommendation anyway.
Correction to buzznut "The most powerful card you could get for this system would be the HD 4670;" well if u ask me i would say HD 3850 is the most powerful card........ the HD 4670 may have 1GB while HD 3850 512MB but the HD 3850 out perform the HD 4670 the reason is this memory bandwith HD4670 128BIT DDR3 and HD 3580 256BIT DDR3