Dual 512 4850's or a single 1GB 5770/896GTX260?

Get the HD5770 for sure

Similarly priced and comparable performance.
DX11 and low power consumption is a plus too  

I really don't care much for power or DX11. Raw frame rates are what I'm after, as I upgrade every 3-5 years or so. Is the performance increase of a 4850X2 over a single 4870/5770/GTX260 enough to negate to negative aspects of 512 megs of GDDR3 memory? I intend to ask for another 4870 under warranty, but if that's not available I intend to ask for something else around the same price/pay the difference.

HD 4850x2 is a beast . But
It has two fans and gets really hot while gaming.
Many buyers complain about fan noise and high temps.

So overall,HD5770,4870 are better deal than 4850x2 despite being a little weaker.
though you won't notice a difference at that resolution  

I said HD 5770 cuz it is very similar to HD4870 in performance.

BTW which PSU do you have?

On noise: Don't care. Keeps the dog away. Impresses visitors. Headphones filter out noise.
On heat: Where I live, in summer, even something like a GeForce 8600 will nudge 90 under load. Half the time I simply open up the case....
So is the performance bump over the 4870/5770 worth sacrificing that much memory? If it's gonna start running out of frame buffer by the end of the year, I'd rather go with a 5770/another 4870.

Just for the sake of reliability get the 5770.
Even that will be able to play games at 1024X768 for many years.

If you don't care about power consumption,Heat dissipation,Noise then get the 4850x2.

Its a good card.Not every piece is noisy.  

Reliability. Now that's something I wanna hear. OK then, will go for a 5770/4870, else if not available, 512 4850X2.

You won't feel a difference with either card. All of them are much much much more powerful than what you need for 1024X768.

Overkill=One step closer to the impossible dream that is future proofing.