For the most part the HD 4850 manages to outperform the 9800 GT, but it's worth noting that the 9800 GT actually comes in about 10% - 15% cheaper than the HD 4850. The other thing worth nothing is that the card not only manages to use less power, but run cooler than the HD 4850.
As far as value for money goes, the card really does sit in line with the HD 4850. While it's slower, it's also cheaper. If you could afford to spend the extra money we would probably suggest you make the move to the HD 4850. With that said, though, we would recommend that you try and get something with aftermarket cooling to help bring the temperatures down.
MSI have done a good job with the card; the cooler is effective, offers good noise levels and the bundle which includes the HDMI adapter that lets us output sound along with a copy of Rainbow Six Vegas 2 allows it to stand out compared to a lot of cards these days which don't include anything more than the necessary drivers and the minimum amount of cables.
If you're looking for a good performing 9800 GT that offers a neat bundle and a game that you would probably play, you can't really pass up on the MSI 9800 GT. While it's not the cheapest card out there, if the game is something that interests you then it offers very good value for money overall.