To clarify, RAM need not to be in pairs, unless you are seeking
to improve the performance of your computer. This can only be done
if your motherboard supports Dual Channel. Applies only to DDR
RAM.
For Dual Channel to work effectively, the RAMs are preferably of
the same speed and size. Example: 2 sticks of PC2700 1GB RAM
Next... different types of RAM may not work.
If you are talking about SDRAM, they will work, but will work at
the lowest possible speed. This applies to motherboards which
support both PC100 and PC133 RAM. If your motherboard doesn't
support PC133 RAM, it might give out errors on starting up.
For example, you have one PC100 and PC133 RAM on your
motherboard. The motherboard will automatically run @ 100MHz, not
133MHz.
For DDR RAM, it's not possible. DDR1 RAM will only work on
motherboards that support DDR1, DDR2 RAM will only work on
motherboards that support DDR2. That's due to the the different
number of pins for DDR1 and DDR2 RAM. Also, the notch on both types
of RAMs are at different places.