Both the PC itself and the form factor are constraining issues. We're talking, after all, about a business-oriented PC from nearly a decade ago. It does have a PCIEx16 slot, but you can only you cards that are low-profile. A 750ti is about as high as you can go (low profile version) and even that will probably be held back somewhat by the rest of the machine.
And even that's presuming the PSU is somewhat competent, something not necessarily the case on OEM builds, especially ones in which the base configuration uses very little power.