I just googled 890 chipset and that was first Mobo that I saw, theres plenty of 790 chipset Mobo's that will run 1600 ram, I just hadn't the time to research a load of them but 1333Mhz should be fine for anything you'll be using it for imo
I run 1600 and theres no great improvement over my Gf's almost identical rig running 1333
We both run Sapphire 5770's for WoW and they run it great on max btw
Couldn't say without researching that myself mate,
I would hazard that theres some benefits to it, some drawbacks but for most applications You should'nt really see much difference between the two, same as 1333 and 1600, most people wont get anything perceptible, its when you O/c the 1600 you'd have more headroom to play with
I use a HD5870 and my friend uses a 9800GTX+ and we both get similar fps in WoW. We both get around 80 FPS in populated areas and play on Ultra settings, the only difference is I play at 1920x1200 and he plays at 1440x900. Nvidia cards can usually turn the shadow settings up with little penalty in WoW. I wouldn't doubt that the GTX460 would outpreform the 5870 in WoW.