Much safer to ditch all the RAM that you have now and purchase new 4 x 2GB, or 2 x 4GB.
As RAM is so cheap (although DDR2 is now higher priced, due to destined to be used for spares only) you are better off just buying 8GB new.
Make sure the sticks are compatible with the motherboard. See if there is a QVL file on the mobo manufacturers site, which will state exactly what brand and type of sticks you should use.
For 8GB in UK you should be paying:
DDR2 8500 £160
DDR2 6400 £100
DDR2 5300 £80
DDR3 Dual Channel 12800 £60
DDR3 Dual Channel 10666 £50