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if you have a mac and want a guarantee that the drive will work in your system, you can get an owc. otherwise, I would just compare the different sandforce drives on the performance level of the owcs and buy whichever is less.
the agility 3 is a bad idea because it uses asynch nand, skip it.
the m4s are in no way crippled though. in fact, if you can find it for 256gb and it's not that much more expensive, I would get.
no idea about intel 520 release date but the sata offerings it has don't look that good to justify the wait. I would get the m4 or the cheapest synchronous sandforce drive available.