Hardware RAM allocation error

So I recently bought a new graphics card and since then I've had problems with memory allocation. I have 4GB of RAM, all of which is treated as usable according to performance monitor, with only 3mb being reserved for hardware, making my machine run slower than it should. If I disable 1gb of RAM (either by plugging the module out of the mobo or through MSConfig) I get 3GB of usable RAM and 1GB of hardware reserved RAM.

I'm on W7 64-bit, so I figured that can't be causing any problems, so I decided it's a BIOS issue. Called up tech support, they told me there were no hardware problems provided the extended diagnostic that you run from a disk that comes with your PC says there's no problems, and there weren't according to it.

So now I guess I'm looking for a way to fix this issue, most likely by somehow remapping the memory. I'm using a Dell XPS 720, which doesn't have a memory re-mapping option in the BIOS, though I presume that it can support more than 4GB of RAM, even though my PC originally came with 32-bit Vista, since the mobo itself is supposed to support up to 8GB of RAM according to the manual.