in my mobo (a MSI K9N SLI Platinum) I've already got a GeForce 7900GS running. the thing is: this GeForce (a XpertVison one, with 512 MB/GDDR3) seems to be extremely rare. I can't find it even in XpertVision's website. so I'm wondering: if I buy another 7900GS, but from (per example) MSI and with 256MB, will the SLI work fine?
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.
Using Coolbits, you can downclock an overclocked model to work with a standard clocked one, or you can disable half of the memory for one that has more.