ya, else you'll have problems making u crazy. BUT you can do as well that.
use 512 but of same manufacturer/type, and in that slot which is other than dual-channel enabling. you can find information in ur manual, cause I am using 875p chipset board and it has 4 slots (all occupied with same DDR's)
i think, if u install one in green and other in the far one, this will enable ur dual channel.
rest u can install 512MB version of same manufacturer and same type. will also wont matter. giving u 1GB ram
Intel Core2Quad 2.66Ghz 12MB L2-Cache
4096MB DDR Kingston Hyper-X
XFX 9600GT AlphaDog
Seagate 2 X 320GB SATA2
LG L246WH Flatron 24"