|A lot of motherboards come with reserved spaces for hardware that wasn't necessarily installed on all versions. ISA slots and on-board sound are often 'missing' too.|
Besides, as mentioned above, the difficulty of getting an AGP slot soldered in it would need to have a bios that supported AGP. Maybe the one you have already supports it. If not, no doubt you have an Intel OEM board and could probably find an AGP bios for that board on their site. You'd use the cmos jumper option for 'recovery' (at least I think they still use that) to install the Intel bios on the Dell.
You're not really green until you're soylent green.