But also remember the speed also matters, because if you have 128bit and run it on quad pumped GDDR5 @ 800Mhz (effective 3200Mhz), and then you have something that would have equal bandwidth to 256bit @ 1.6Ghz/3200Mhz.
So if you see something like the HD4770 with 512MB of GDDR5 running @ 800Mhz it would have much greater bandwidth than the HD4830 with 256bit running @ 900Mhz/1800Mhz ; although memory bandwidth isn't the only thing, but for this question you need to make sure you consider that, since the answer by xcO is assuming all other factors are the same.
Also I just noticed xcO's errors too (don't know if it was there when I first replied before his edit);
xxx-bit is the "speed" of the ram.... the other would be the amount of memory
Actually xxx-bit is the WIDTH of the pathways between the memory and the onboard memory controller when added together.
GDDR5 also sends twice the date down each pathway that GDDR3 does (it's like double-double data rate , but not QDR or XDR), so it's like doubling the speed, but the memory is still clocked at a lower rate, but the bit # talked about is the width of the pathway for the memory, which is like saying what's the number of cars that can pass through a given section of town if it is so many lanes wide, and such speed. The bit # tells you the number of lane width, the Mhz tells you their speed, and the MB/GB number tells you the size of the parking lots at both ends that these cars go to & from.
speed is more important then the amount... 1 gig is unnecessary until the very high end, but yet again, the very high end doesn't use 128 bit
HD4770 uses 128bit and it will stomp the 512 bit HD2900 every day of the week and 3 times on Wednesday (ladies night! )
waht you really want to look at are the benchmarks, you cant buy a video card alone on memory