Well if you're talking about dual channel rams, which is a pair of male sheep...wrong forum

Dual Channel RAM however is a method of improving memory bandwidth.

A standard Synchronous Dynamic Random Access Memory module (SD-RAM) has an interface that's 64-bits wide. This means that every clock, it can transfer 64 bits, or 8 bytes. A module that runs at 200 MHz Double Data Rate (DDR400), clocks 200 million times per second, transferring 3.2 GB per second. This performance can be improved by adding another channel - another logical memory subsystem - in parallel. This increases the interface to 128-bits wide, allowing up to 6.4 GB per second. You may have heard of graphics cards with 128-bit and 256-bit memory; this is dual channel and quad channel respectively. In the past, there have even been quad channel SDR chipsets, but such is not necessary today with high clock speeds.

Intel's i865/i875 dual channel solution is symmetric, which means that each channel must have exactly the same number of banks and chips, which also implies that it must have matching pairs of memory. If an uneven quantity of memory is installed, the memory controller falls back to single channel mode. Realistically, there are few circumstances where this is unavoidable. In your case, a total of 2.5 GB will offer nothing than 2 GBs does not, except that your memory bandwidth will be halved, hurting performance; gaming especially.