Do it yourself. Open the case and you'll see another stick of RAM the same as your new one, stcking out from the motherboard at a right-angle. Next to it should be a spare slot with white clips at either end.

Earth yourself by touching the case or an unpainted part of radiator pipe (to discharge any static built up that can fry your RAM or other components).

Push your RAM firmly into the slot (it will only fit one way as there is a cutout about 2/3 of the way down) and the clips will move into place. Push the clips in to make sure it's held firmly in place. Don't close the case yet.

Restart and see if it boots up as normal. Check to see if it recognises the RAM by checking your system properties. If it works OK do the case back up and you're all hunky dory.

Once you've done it you won't believe you considered paying somebody.