Well... let's talk raw pixel pushing power...
The G1 (1.6) has a resolution of 320 x 480 = 153,600 pixels.
The Droid (2.0) has a resolution of 854 x 480 = 409,920 pixels.
Therefore, the Droid has around 2.7x the pixels as the G1, and hence (generalising here) it needs 2.7x the graphics processing power to deliver the same frame rate.
Both the Droid and the Nexus 1 have graphics chips that are, on paper, around 2x faster than the G1. But, the Nexus 1 has a more powerful processor, so appears slightly faster. Anyway, my point is they're on a pretty level playing field in terms of frames per second. This can be shown by running Neocore to benchmark the graphics on the G1 and Nexus 1. Someone has already done that here: http://android.modaco.com/content/t-mobile-pulse-pulse-modaco-com/298879/neocore-pulse-vs-g1-vs-nexus-one/
G1 = 24.5fps
N1 = 26.7fps
So, in terms of fps (which is what matters), the N1 is almost identical to the G1. I doubt graphics are a factor in delivering Android 2.X to the G1.
Also, you will notice the some handsets, like the HTC Hero, have been confirmed as getting Android 2.X, even though they have identical graphics chips to the G1. The HTC Hero, however, does have a 256MB ROM chip, as opposed to the G1's 128MB.
In fact, the only phones that have already got, or have been confirmed as getting, Android 2.X all have at least 256MB ROM chips. All the ones that haven't been confirmed as getting it have 128MB ROM chips.