The differnce between 1 DIMM and 2 DIMM of 512mb memory

[Apokalipse]

I suggest you put in the 512MB in with the RAM disk you already have - assuming you bought an extra RAM disk to put in - so you get more than just the 512MB.
even if you can't run the RAM in dual channel it should still give you better performance, because the OS runs better with more RAM, except when you get over 1GB, but you don't have to worry about that.

 

[Kangaroo]

DIMM - Dual Inline Memory Module

Almost everybody's computer uses DIMM, DIMM is just the name of a socket. 2 DIMM memory probably just takes up 2 of these DIMM sockets, whereas 1 DIMM occupies just the 1 socket leaving the other sockets empty and able to take more memory.

 

[Elbatrop1]

even if you can't run the RAM in dual channel it should still give you better performance, because the OS runs better with more RAM

I agree.

For your (Sandy) needs, 512MB should be just fine.