In my understanding of dual channel, is that it accesses both ram sticks at the same time,
Precisely, instead of your computer using up one 512mb stick and then going to the next, it accesses both at the same time which results in anywhere from 5%-20% gain in performance. Don't be fooled though, dual channel won't make a HUGE performance increase by any means, but every little counts.
Would 4 X 256mb = Maximum 1gig performance?
No just filling up more spots, completely pointless. Stick with 2x512mb.
Read that regarding top overclocking RAMs printed circuit board designs. Such RAM as:
OCZ Platinum Rev 2
Patriot XBL Series
Are the best overclocking RAM you can get.
Benefits of overclocking RAM you say? More bandwidth for your RAM to use and have accessible to programs. If you went from DDR400 (200MHz RAM) up to say 250MHz DDR500, you would see a nice performance increase in boot up times, programs loading, few FPS in games, and as long as you keep your CPU 1:1 with the RAM (meaning your CPU is 250MHzx(multiplier) = (final clockspeed) then your CPU and RAM is communicating at the same speed which = an overall nice performance boost in system performance.
Say you had a 2.5GHz machine...You had a 400FSB CPU...so in the BIOS it'd say 'CPU FSB = 200' because you multiply the 200x2 = 400 because ram is DDR (DOUBLE data rate) double being the keyboard...so 200x12.5=2.5GHz and your RAM being PC3200 is DDR400....decent system of course, but if you did 250x10 = 2.5GHz and had your RAM up at 250MHz DDR500 the system would be the same CPU clock speed, but overall would be a lot faster thanks to the faster RAM