Despite the non-impressive 900MHz basic frequency, GDDR5 based Radeon HD 4870 will have an impressive bandwidth and speed of 3600MHz effectively.
The 512Mb GDDR5 chips that ATI uses on its cards with 900MHz frequency have 3.6GB/s throughput or double what GDDR 3 or 4 have at the same 900MHz speed. You can blame Quad data rate for such a throughput. Depending how you want to write it, it's either 900MHz times 4 or 1800MHz, which is the GDDR5 dual data rate times two cycles. The important thing is that both would add up to an impressive 3600 number.
ATI’s Radeon HD 4870 512MB card has eight 512Mbit chips on the card, while 1GB cards will get the whole sixteen chips, but this will probably happen at some later date due to the high cost of GDDR5 memory.
You can see that Radeon HD 4870 will end up with almost twice as much bandwidth than Radeon HD 4850 and we are talking about a number close to 115GB/s, at least if our math worked well this time.
The card is out tomorrow.