If you're counting paychecks I'm not sure why you would want to buy into an intermediate upgrade, it's essentially wasting money. Simply, the HD 4890 will perform lower than the 5850, and it will not support DirectX 11. It's a great card, don't get me wrong, and will significantly increase your gameplay over a 9800GT; I'm using one, but the 5850 is the obvious choice. The only problem with the 5850 is finding the thing. :/
The main objective is what is gonna save you money and what is gonna be viable. If you spend money on a 4890 now I really wouldn't advise buying/carrying it over for a medium/higher end i7 build because with that processing power you want to expect to have a card that can keep up as well. As such, the 5850 is much more viable to carry over into your i7 build, and if you need more power you can add a second one (if you get a CrossFire capable motherboard) later which is about equal to a stock 5970 in most games.
Like has been said before, there's a noticible difference in card price between the HD 4890 and HD 5850 -- about $100 or even more. Regardless, even if the card ends up only costing $175, that's $175 more you're gonna have to spend that you don't have when you're building your i7 system. If you really, really have to have an intermediate upgrade I'd recommend an HD 4870. An HD 4770/4850 performs too close to a 9800GT to spend money on, but I still think you should just wait.
Well I will have this set up as a second computer for friends to LAN with after i get my i7 rig, so i want it to keep up with current dx10 titles that we play/show some epeen around my friends.
Not keeping your loaner computer up to date will be good motivation for your friends to spend money on their own awesome computers to bring to a LAN.
If they don't care that much I doubt they'll care to notice the video card in your old one.
As far as my i7 rig goes, thanks for the input but the extra money that I would save wouldn't be enough to allow me to get my i7 rig any faster. Just counting down paychecks
Also the 100 dollar 4850 is a wonderful card, or so I hear, but would the performance be very noticeable coming from my 9800gt?
$150-200 is enough to make a noticible difference in any build.