Probably the best site would be ATi's own.
I'm doubtful these will be out very soon at all, as they're having a hard enough time getting the 256MB versions out, and supposedly a shortage of DDR3 is at least part of the cause, and so them wanting to pu twice as much on a single card isn't going to be very good for them from a business standpoint.
Also, isn't R520 just around the corner. I think I remember hearing its supposed to debut around E3 again (mid-May).
Who knows for sure though.