I know very little about ATI cards, but you should take a look at this chart. It gives which cards can be xfired together, although has not been updated to include the 4890 yet.
when i am not doing graphics intensive work both cards will be consuming power or only one card
Both cards will be consuming power. If the applications you are using support corssfire, then you will use both cards. If it does not, then you will be using one card and the other card will idle. Idle cards use less power than cards in use but they still use some power.
They'll work, but the faster card will in essence be limited to the speed of the slower one. There's no real point on running a 4850 and a 4890 together because of that. Just run a pair of 4850s - it'll be more than enough.