XPS-1530 Video Card Upgrade?

I have a 256 MB nVidia graphics card in my XPS-1530 system (the GeForce 8600 which originally came with this system).  Is it possible to upgrade this card to the NVIDIA®SLI™Dual GeForce®8700MGT with 512MB2 GDDR3 Memory or a similar 512 MB card?  I've asked Tech Support but they haven't answered.  Thanks.