The added RAM won't make that much of a difference as far as I can tell. Its already got enough to do most things, even at eyefinity resolutions, so no worries there.
And no- ATI did not do the same thing with the 4k series. The 4890 is SIMILAR to an overclocked 4870 (NOT the x2), but is not just that. The 4890 is actually a slightly updated GPU Core. It has basically the same SPECS as an overclocked 4870, but with a with a core that was optimized from what the 4870 used. Also- the 4890 was a new refresh released BY ATI. The so called "5990" is NOT being released by ATI. It is being released by Sapphire, and is using the exact same GPU cores and specs as the 5970, just with higher voltages and clock rates and a bit of added RAM. It is litterally an overclock that you could do at home on a 5970 with the right software and a good cooler. It truly is a 5970 with more RAM that Sapphire has overclocked and CALLED the "5990" Just like the ASUS Ares model of the 5970 that is supposed to be coming out ( http://www.fudzilla.com/content/view/17831/34/
). NOT a new ATI card, just a new non-reference Sapphire 5970.
Edit: Fixed URL