Matrix 5870 2GD5 - dxdiag issue

Hi everyone (first post, so bear with me)  

I've had a few gpu problems and have gone from a XFX HD5870 to an ASUS Matrix EAH5870 2GD5.

Now, when I go into dxdiag, it says my approx video memory is 996 MB and is should say 2048 MB. So I uninstalled the ati 5xxx series driver, thinking that it might help if I were to install the driver from ASUS's site. After uninstalling the ati driver, I went to check what dxdiag might say, when no drivers were installed, and it was suddenly detecting 2048 MB, which is what the card should have. After installing the asus ati driver, dxdiag is back to showing only 996 MB yet again.
I wasn't sure if this really had any effect or not, as non of the games I played thus far, stuttered or lagged. However, then I launched Prototype. Prototype wont allow me to set my screen resolution to 1440x900 nor set AA any higher than x2, which was no problem with my previous HD5870 card. I read on activision's site, that Prototype wont give options for my desired resolution, if I have less than 1024 MB of video memory...
So this is where I'm beginning to think that dxdiag not showing the correct amount of video memory, might have some drawbacks after all.

My question to you guys: Is there any known way to correct this problem, where dxdiag doesn't detect the correct amount of video memory on your card?
I did a complete reinstall of windows 7 ultimate 64 bit and that didn't do the trick.

Pc specs:
CPU: Intel i7 2.66 MHz
Motherboard: lntel X58 lCH10 SOCKET 1366 DDR3
PSU: 750W Cosair
RAM: 12 GB Corsair Tri-channel 1300MHz
HDD: Samsung 3S spinpoint 1TB