Yeah, dual channel is basically a small increase in data transfer, but it benefits latency more than anything else, which translates into stability.
However, since 256MB sticks generally have lower timings and overclock overall better than larger sticks, it would probably be somewhat of an advantage over 2x512MB.
There is no concept of 'quad channel' at all, like shoobie said.
if you had 4 sticks of 256mb your computer would see it as 1gb of RAM in dual channel, that's it.
However, since 256MB sticks generally have lower timings and overclock overall better than larger sticks, it would probably be somewhat of an advantage over 2x512MB.
The size of the memory has no effect on the timings. It's the particular IC's used on the RAM that determines that, so although the statement 2x256 tend to OC better is true the '256mb sticks have lower timings' is not.
Dual-channel just means that the two sticks will work well together. The performance increases are often negligable at best.
Dual-channel is not going to make your RAM run twice as fast or make it seem like more RAM or anything like that. There's simply a small 5-20% speed increase, at best, because the RAM sticks communicate more easily than non-dual-channel setups.