i've been searching for a review for 6670 with ddr3, i could only find a legitreviews article of a cyberpower pc where gpu-z says that the 6670 in that pc has gddr3. it didn't have crysis benchmarks. i did notice in the gpu z screenshot that both ddr3 (gddr according to the gpu z screenshot) and the gddr5 version has same gtex/s, same pixel fillrate but different memory bandwidth - 25~ gb/s for (g)ddr3 version vs 64 gb/s for gddr5 version. 5670 1 gb gddr5 has the same memory bandwidth but slightly slower gpu compared to 6670. from this info, i'd say that the 5670 with 1 gb gddr5 will perform slightly better overall, between the two you listed.
edit: a 6670 1 gb gddr5 will perform slightly better than the 5670... a 5750 or a 6750 1 gb will perform even better.
DDR5 over DDR3 does not always translate into an increase in performance. For higher end cards, it makes sense to go with DDR5 since the GPU is actually strong enough to warrant the extra memory performance. Faster memory doesn't mean anything if the GPU isn't capable of processing enough data to actually use the extra memory performance.