What power supply in particular? Can you link to it? Also what resolution are you running at? A 4850 is far from rusty, if you have a crossfire capable board you can get a second one and have performance similar to a 4890.
You can put your system into this component by component and it will give you the required PSU needed and what the new graphics card will up the PSU by.
Generally these calculators tend to error on the side of caution and I would have to say that is probably a good thing. I would rather have more power than needed than not enough.
As a side note, depending on what 4850 you have and the 4890 you would like to get, you may or may not see a difference as far as FPS other than that at higher resolutions you might gain 10-15 FPS at best.
I have a 4890 and my dad a 4870. The difference between our games is negligible. Kinda second guessing getting a 4890 over the 4870.
What's your budget?
400$, wait for 5870.
150 and lower, just crossfire a second 4850. Though you can only do this if your PSU is a good brand (corshar, CoolerMaster, etc), and your mobo supports it.
It seems i have alot more to learn than I thought...
I didnt know you could run 2 different cards in xfire.
So I have a few rather stupid questions...
As mentioned I play at 1920 x 1080. Would I benefit most from buying a 4890 with 1gig ram or another 4850 with 512MB? I just learned an extra card wont give me more ram and as far as I understand amount of ram is kinda important at the res i play.
A big issue for me is that in Norway (as far as I can see) a new 4890 costs almost the same as the 4850...