No it's not. GDDR3 is not any faster per MHz than DDR2 SDRAM. GDDR4 is not any faster per MHz than GDDR3. GDDR5 (which should probably really be called GQDR) has 2x higher bandwidth per clock than DDR, DDR2, DDR3, GDDR3, & GDDR4 memory.
The main difference between DDR, DDR2, DDR3, GDDR3, and...