TT,
Try re-seating (extract and re-insert) them both first to see if its an oxidised connection. If that does not cure it then you can try removing one of each stick at a time.
Typically you cannot mix DDR400 memory with DDR333 (especially in the same memory bank). If your motherboard can cope with DDR400 you would be slightly better off binning the 2x256 and using the single 400mhz - and if possible buying another 512mb or 1gb
Cheers