Hey Nameon,
To make this decision you will need to consider what you need the card for.
If you have a really high resolution monitor (and I mean 1080p+) and need the maximum performance at high resolutions for it the 4890 is a more powerful card. However the 4890 also runs hotter, requires a much more powerful power supply, and doesnt have direct x 11. Personally, I would go with the newer technology (thats cheaper too) over the extra muscle.
You should post your monitor and other system specs for reference, as well as what type of gaming you do.
(Also, ATI will be replacing the 4890 with the 5830 at around $220ish very soon.)