memory on a gpu is directly aimed for the resolution of the screen, and does not justify the "speed" of a gpu.
one of the most known gpu vs mem topics there is, is the gtx295 (1792mb) vs 4870x2 (2gb)
the 4870x2 has more mem than the gtx 295 and had a clear advantage over the gtx295 at 2560×1600 res, but still fell against the gtx295 for having 2 weaker gpu's.
there really isn't a way to just simply tell which is faster, so the best idea for you to do is look at previous benchmarks.
and for a random fact, GTX460 is faster than a GTX465. as the GTX465 is nothing but a crippled GTX470 that didn't meet certain criteria. which is why that is the ONLY model of the fermi line up that does not have a refference model... lawl
