I'm not a big fan of OCZ.
BTW, Crucial 2100 is all 2.5, but mine runs just fine at the most aggressive settings (2,2,2) and 1T.
If you are using an AMD board, you will have much better luck with registered. If you are running a VIA board, don't bother. Not only will you get worse results, but some of the boards only operate with ECC if all of the mem registers are filled.