I got this from the NVIDA Forum, maybe it helps or you may have already read it. On a side note a GForce web site it says you can mix memory sizes but recommends CoolBits to lower the memory of the higher card to the lower memory.. in you case both would run at 2GB.
Can I mix and match graphics cards with different sizes of memory?
This is not recommended, so it would be better if you could consider the answer as a “no”, but it is possible using CoolBits. Though at the end of the day, if you currently own an 8800GTS 640MB you should purchase a second 8800GTS 640MB. However, when 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 the effective memory size for each card becomes the smaller of the two memory sizes. So an 8800GTS 640MB paired with an 8800GTS 320MB would operate the same as 8800GTS 320MB SLi.
Step 1. Click on your Start Menu in the Windows Task Bar and select Run. When the command box opens, type "regedit" (without the quotes) and press OK. With the registry editor open, go to HKEY_LOCAL_MACHINE \\SOFTWARE \\NVIDIA Corporation \\Global \\NVTweak (expand the menu using the "+" expansion buttons to locate this menu item). Click on "NVTweak" in the menu. Next, right-click on the empty space inside right window and choose "New>DWORD value". Name the value "Coolbits" (without the quotes).
Step 2. Within the Registry Editor, right click on Coolbits and choose Modify. Change the Value data to "18" (without the quotes) and click OK. Exit the Registry Editor.
Also you might have a look at this page to see if it helps you... http://www.overclock.net/t/1150443/how-to-remove-your-nvidia-gpu-drivers/0_20#post_15432476Edited by Conners - 4/29/13 at 8:51am