YES, going from 512MB of RAM to 1 gig will make a considerable difference in gameplay
No this is not true, well it is but its not. Going from 512mb to 1gb of the same ram will not really result in any game performance. it will give better performance in video editing though. Now he said major brand. It depends on his brand now, its cas/timings, etc. Corsair XMS Extreme Memory Speed Series is not neccecarly alwasy faster than some other brand, this pack is just made for overclocking specifically and good performance. But if he has 512mb of ram with the same statistics of that 1 gb it will not give a huge performance boost.
timings are very difficult to explain. Just think of is as the lower they are the faster.