Quote Originally Posted by Glow9 View Post
Curious maybe someone can explain but why does the ATI card only have 256-bit memory interface, It has a bit more ram than the GTX yet the GTX has 448-bit memory interface?
Quote Originally Posted by largon View Post
To be precise;
HD4870 has eight GDDR5 chips each 1Gbits (128MB) in capacity yielding total of 8×128MB=1GB and since each chip has a bus width of 32bits the whole bus combined is 8×32bit=256bit.
GTX260 has 14 GDDR3 chips, each 512Mbits (64MB) totalling 14×64MB=896MB, total bus is 14×32bit=448bit. GTX280 has 16 chips and as such for the calculations: 14->16.

-Snip-
To add to what largon has said, that the 4870/4890's are using GDDR5, where as the GTX's are using GDDR3.

The GDDR5 is quad pumped, so 900Mhz has four bits per clock cycle = 3600Mhz effective. 3600Mhz x 256 bit = ~921600Mbps = 900Gbps.

The GDDR3 RAM is only two bits per clock cycle. So 999MHz x 2 = 1998MHz effective clock speed and 1998MHz x 448bit = 895104Mbps = ~874Gbps

So you see, the total actual memory bandwidth is much more similar between the cards, its just that they use different methods to get there.

Just for the hell of it I will also mention that from a card manufacturing point of view, the lower bus width (eg, 4870) is easier to implement - there is half the number of wire/traces to cram onto the circuit board with 256bit bus vs. a 512bit bus. This leads to a cheaper to produce card and is possibly part of the reason why the radeons are cheaper. It is also the technique that has been successfully used to make many a mid-range card. Create high end card, let manufacturing processes mature and cheapen, chop memory bandwidth in half and use a cheap but matured memory technology running at full steam and you have a decent performing mid range part. Anyway, enough lecturing, someone will correct me soon.

-Muunsyr