I'd be careful moving to the m9750 power supply as I think it has a different output that could cook the charging system and other components on the motherboard. The power supply doesn't just power the vid card...
I'm no expert on the issue, but I've seen posts with people looking for replacement AC adapters for their m9700's, and they've been told not to use the m9750 power supply as it has different voltage and amp numbers.
The issues are: whether the gpu slots on the m9700 can supply enough power for the 7950gtx, whether it'll fit, and whether the cooling is adequate.
I just looked at the PSU for the M9750 and the M9700. The main difference I see is with the current: M9700's PSU is rated at 7.89A while the M9750's PSU is rated at 9.5A. Both PSUs are also rated at 19V (Hence the M9700 being rated at 150W (19V x 7.89A), and the M9750 PSU being rated at 180W (19V x 9.5A))
The same issue occurred with Dell Inspiron E1705 owners who upgraded their GPUs to a 7900 or 7950 GTX. The original PSU was a PA-10, which is rated at 90w (19.5V x 4.65A) and when owners used their machines with the GPU + the PA-10, their performance was marginalized (downclocked) because the system couldn't draw enough power. The PA-13 (19.5V x 6.7A), which came with the Inspiron XPS models and Precision M90 were used on these systems and were able to fully run the replaced GPUs along with having no issues with overheating, bad charges, etc. Matter of fact, when I had my M1710, I bought a PA-15 (19.5V x 7.7A; from the Inspiron 9100 that consumed more power, XPS Gen 1 and XPS M2010) for it and my system worked fine for a year, till I sold it.
Now in terms of fit and cooling adequacy, again, the cooling system on both chassis are similar (both accomodate MXM-III GPUs), and the structure of the GPUs are identical. When I took out the 7900GS from the M9700 to sell, it looked identical to the 7950 GTX in the M9750. But if the system is able to draw more available to the GPU, then it would as a result of a higher power GPU.