depends... lets say you have a mobo that supports up to 4gb of ram... you get 2 512s then it will be faster but u will have only 2 slots left... u get 1 1gb and you will have 3 slots left... so wut happens with having 2x512 is in mebbe a year and 1gb gets to be pathetic and 4gb gets to be pretty standard so you want 4 gb... well then u have to take out the 2 512 sticks get 4 1gb sticks... opposed to keeping the 1gb stick and getting 3 more 1gb sticks l8er on...
so 2x512 is a little faster, but 1 1gb is easier for upgrading...
the only environment where having 2 sticks of an ammount eqauling what it would vs a single stick is a dual-channel one, so unless your going to have dual channel, its better to buy a single stick