DDr3 1333MHz is the good enough. For anything better you need a good MB that can support it. I recommend some Kingston Hyperx 2X2GB modules (good for overclocking or tightening the timings) and it's not that expensive.
The AMD website says that the phenom ii x4 955 supports up to 1333mhz but my motherboard can support 1800(O.C.)/1600/1333/1066 MHz so can I use any of those or do I need to keep it with 1333 for my cpu?
It's not needed to keep it at 1333 but let me just warn you that 1600 might give you some headaches with an amd cpu.
i personally recommend the 1333mhz ram. 1600 won't give you any visible performance boost and buying 1333 with some tight (the lower the numbers the better) timings is much better and can sometimes work faster if the timings on the 1600 ones are loose.