The 8800GTS 640mb is weaker than the 8800GT so no it won't be between those 2, it will be lower than either.
Thank you kind sir for the correction. I need to stop reading these threads way past my bedtime, lol.
Its apprx 20% weaker than the GT so a rough guestimation would be apprx 20% weaker than the GT SLI score. Its not completely accurate but its the best educated guess based on the chart...