never seen it called "base unit" before, but thanks to the wonders of google, i think you're referring to your case?
and by "power unit", i'm guessing you mean your power supply (psu)?
if yes, then i think i understand your situation; you probably have an m-atx case, which means only m-atx mobos are supported.
ok, so i'm not sure where you're able to shop from (your location?), but i just skimmed through newegg
here, and there are no m-atx lga775 mobos with 2 x16 slots, only atx.
only getting a larger case would allow you to use an atx mobo. or not using a case at all, but that might be impractical for you.
another option could be adding another gfx card, but im guessing you're already using a pci-e card which is currently populating the only x16 slot on your mobo. but i think your mobo should have atleast 1 pci slot, and there are are gfx cards with a pci interface you could also use though, to run another monitor off of.