Actually you probably will have better performance if you can Dual-channel them.
What CPU are you running? choices are:
939-pin Athlon64 512KB L2 Dual-channel capable
940-Opteron/Athlon64 FX-51 1MB L2 Dual-channel capable
754-Athlon 64 1MB L2 Single Channel only
Running a 754-pin you might be better off going with one stick since you can run 1T CMD quite easily and dual-channel is not an option. 939 definitly go two sticks of 256 since dual channel is supported. If you are unlucky enough to have a 940-pin then you must get at least registered ram and to be safe get it with ECC (a must with opterons from what I heard even if it is not on - FX-51 does not need it). 939 and 754 have the joy of running unbuffered ram and do not support ECC.
Myself I run a 1.4GHz opteron 240 (it was the only model in stock at the time) with two Corsair registered ECC DDR400 512MB sticks in Dual-channel and the setup runs great with no errors what so ever. Although the price for registered and also ECC at DDR400 was not so pretty the reliability is awesome.
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