They are all compatable with your PC's specs so dont worry about that.Out of the three the 550 Ti is the slowest and the HD 6850 is the fastest.So... if i were you i'd choose the HD 6850.If you are worried about power consumption go for the HD 7770!
Hope i helped.
yeah! i just bought the Sapphire HD6850..
i saw a big improvement on my games.. on skyrim, NFS, etc.etc..
but in NBA2k12.. i encountered some slight lags while playing..
i think it is because i only copied the NBA2k12 Folder of my friend and pasted it on my C: ... then presto! i can play NBA2k12 without installing it..