straight from the website...
What Graphics card should I get?
If you're building an NVIDIA SLI-ready PC and starting with one graphics card now, with plans to add a second one later, you're probably worried about being able to find the EXACT same graphics card to make a matched pair. Fear no more! With NVIDIA's latest driver series, you guys can mix and match NVIDIA graphics card vendors, memory clocks and GPU core clocks - as long as the GPUs are the same. Here are some examples of what combinations would work, and some that would not:
EVGA GeForce 6600 with a BFG GeForce 6600 GT = WOULD NOT WORK (different GPU)
ASUS GeForce 7800 GTX 512 with an MSI GeForce 7800 GTX 256 = WOULD NOT WORK (different GPU)
Leadtek GeForce 7800 GT with a PNY GeForce 7800 GT = WOULD WORK (Same GPUs)
Also, it's important to note that you don't need to have a motherboard from the same manufacturer of your graphics cards. For example, you could have an ASUS motherboard, paired with an XFX GeForce 7800 GT and a BFG GeForce 7800 GT. As long as they are all NVIDIA SLI-Certified parts, you're good to go!
With the flexibility of the most recent NVIDIA drivers, you don't have to worry about being able to find the EXACT same make and model of your existing SLI-ready graphics card when you upgrade to SLI - you just need to worry about finding the same model (same GPU). So if you buy a graphics card from XFX featuring a GeForce 7800 GT, you won't have to necessarily buy an XFX GeForce 7800 GT when you decide to upgrade - you can do so using ANY NVIDIA SLI-ready graphics card featuring a GeForce 7800 GT GPU.
Considering that and the examples...it is hard to say, but I would go with most likely not...