Even though the specification of Radeon 4890 is set to 850MHz core clock, some partners will release factory-overclocked versions of the card. They will work at 900MHz for most, but someone might actually even go higher. MSI's R4890-T2D1G-OC is the first overclocked HD 4890 we've come across, and it runs at 900MHz GPU, while the GDDR5 memory is clocked at 1000MHz.

If you do some manual overclocking you should be able to get close to 1000MHz and with water, over 1000MHz should be possible.

Naturally, more you overclock, the hotter the chip gets but this should be something that RV790 can cope with. However, GTX 275 will also come overclocked and that will be a tough competitor for RV790, overclocked or not.