You can mix cards for SLi as long as they are both the same GPU, reguardless of brand. i.e.: MSI's 7800GT & eVGA's 7800GT CO will work in SLi. But you have to use 8X.xx nVidia drivers.
Quote from Mad Mod Mike off SLiZone:
Mixing Cards: You can use a GeForce 7800 GTX from manufacturer ABC with a GeForce 7800 GTX from manufacturer XYZ. However, you must still use two graphics cards that have the same GPU. For example, a GeForce 7800 GT must be matched with another GeForce 7800 GT (not with a GeForce 7800 GTX or a GeForce 6800 Ultra). Likewise, you cannot mix the GeForce 7800 GTX 512 with the GeForce 7800 GTX. Look
here for more information.
This may also answer a few other questions.
Mad Mod Mike's Top 10 - Most frequent questions about SLi:
http://www.slizone.com/object/slizone_ask_mmm009.html
Link for 81.95 drivers:
http://www.nvidia.com/object/winxp_2k_81.95.html