Note that the new AMD Athlon 64 FX processors run DDR RAM in dual-channel mode, but the standard Athlon 64 processors, which use a Socket 754 motherboard, can only run it in single-channel mode. Only the Athlon 64 and 64 FX processors that run on a Socket 939 and Socket AM2 motherboard can run the DDR (Socket 939) and DDR2 (Socket AM2) memory in dual-channel mode. Intel Pentium 4 processors can only use DDR or DDR2 RAM in dual-channel mode if the motherboard's chipset supports that mode, because unlike the Socket 939 and Socket AM2 Athlon 64 processors, they don't have a built-in memory controller
Thanks DieSse I'm afraid my ram modules are the bog standard generic ones, ( although matching) and not on the list from your link,so it's probably to do with that. These are ELIXIR and are currently £8.80 per Gig so probably not the best.