Yes, but it basically disables the "extra" memory on the 2nd card.
From Nvidia's SLI FAQ:
Can I mix and match graphics cards with different sizes of memory?
While it is not recommended, NVIDIA does offer this flexibility using Coolbits. When purchasing a second graphics card, you should try to match the memory size so that you are ensured full value and performance from your purchase. For example, if your first card is a GeForce 6600 GT with 128MB of memory, you should purchase a second GeForce 6600 GT with 128MB of memory. However, using Coolbits (value set to 18), you can force both of the cards to use the lower of the two memory sizes and operate together in SLI mode. When dissimilar memory sizes are enabled to work together using Coolbits, the effective memory size for each card becomes the smaller of the two memory sizes. Instructions to enable this feature can be found here.
So, unless someone is giving you one for free, I wouldn't spend the money that way. Unless you want to run 800x600 at screaming FPS. The video mem size would limit you severely at higher resolutions, I'm thinking.
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