I wouldn't worry. The SGX 540 GPU is plenty powerful, and it should have excellent ICS support, as the GNex also uses the OMAP4 chipset and had the same SGX 540 GPU.
I wouldn't call it an aging GPU considering Google's flagship device uses it.
EDIT: Actually, it should perform pretty much on par with your Droid 3 as far as gaming goes. So you should already have a pretty good idea. A Razr and a Droid 3 run on the same platform, the Razr just adds 200MHz CPU speed and a LTE radio while omitting the keyboard and GSM capabilities.
On the aging GPU, Google's formula for Nexus phones isn't cutting edge technology.
I was actually wondering does the 200 MHz increase in CPU speed or the extra 512 MB of RAM (or both) make a difference when playing intense games. Everytime I play those games on my Droid 3 they render the phone unusable requiring a battery pull after playing for more than twenty minutes.
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