does ddr II memory have to be used in pairs?

No.

Many boards that support DDR2 also support Dual Channel but the two are not necessarily linked.

One stick of DDR2 will work fine as long as the board supports DDR2.

I have a dell dimension 3100 with 512mb ddr2. Don't know whether its 2x 256 or 512 or whether its pc3200 or pc4200

Memory

Type
400- and 533-MHz DDR2 unbuffered SDRAM

Memory connectors
two

Memory capacities
256 MB, 512 MB, or 1 GB non-ECC

Maximum memory
2 GB

http://support.dell.com/support/edoc...n/sm/index.htm

This scan will also help:

http://www.crucial.com/systemscanner/