Go fetch
GPU-Z, install it, run it and check what it says about your GPU. Or RivaTuner or AtiTools or some other GPU-related tool that lets you observe the properties and settings of your card. It's a pretty much non-bribable in that regard, it should tell exactly what your GPU has eaten...
It is likely that both cards are set to borrow some of your system RAM ans use it as extra video memory as default *points finger at DELL* especially since both are budget cards... although especially for the 256MB one, I don't think it's actually necessary to dedicate system ram for it in addition to it's own memory. Donating system RAM for GPU usage is OK if you have *plenty* of RAM. If you don't, you might want to check in driver settings if you can find anything like that and possibly disable it, it'll free more system RAM for program usage which will likely end up improving performance more than using it as video memory. Anyway, if I'm not completely wrong, DXDiag only tells you how much VRAM is usable from the POV of the operating system, it doesn't separate the card's own and borrowed system RAM in that regard AFAIK... so it's the most probably reason for this.
Other possibility is some driver conflict/bug, and the least likely possibility (especially since two different cards are doing this) is that someone in the manufacturing facilities has screwed up to your advantage and the cards actually have more memory than bargained for. It's very unlikely though, but screw-ups like that in smaller scale aren't unheard of - I personally have an XFX GeForce 7600 GT XXX-edition, which is supposed to be factory overclocked core to 590 MHz, but instead it's overclocked to 650 MHz, so I actually got more than I bargained for, and I'm not complaining.

EDIT: Also, this conversation might better belong to GenDisc or Gaming Discussion since it doesn't directly have anything to do with FreeSpace.

EDIT2:
INFERNO! Our Last Best Hope for FreeSpace3
You might want to rephrase this sentiment. Not only does it open the whole FS3 can of worms and whether or not anything community-made could or should be called FS3, it also is kinda inconsiderate towards all other campaigns and mods that are
not Inferno. Some might find it even downright insulting.