The 9800gtx you linked to is pretty much an overclocked 8800gts, so they will perform pretty similarly, but the 9800gtx will be slightly faster.
A lot of people say that SLI isn't completely worth it right now, because in some games it might not scale well. So it might be better to just get one faster card that will be fast in all games. I'm not sure how the 8800gs in sli compares to these other cards performance wise though.
The 4850 is about equal to the 9800gtx, and a little faster than the 8800gts. Since it is the cheapest one, and the best performing of the three, I would suggest getting that one.
Either way, I'm sure you'd be very happy with either the 9800gtx, 8800gts, or the 4850.
the HD4850 is definately the best GPU in that price range. i just got one myself and it is very nice.
with the fan speed at about 40%, the noise is barely noticeable and it cools the GPU perfectly fine. if you get the asus card, like i did, they include a simple utility that monitors the GPU and allows you to easily modify the fan speed settings.
it's amazing and yes, the fan fix is ridiculously simple and works really well.
and crossfire 4850s generally scale really well and will last you much longer if you want to spend ~300.
I was looking at the 4870 and I heard that a 4870x2 is coming out? WOuld it be worth it to get the 4870x2 instead of just the 4870 or get 2 4870's and crossfire them (but Im thinking that this defeats the purpose of getting a 4870x2)? What I was also thinking of was getting a 4870x2, then in a few months get another and crossfire them. Im also thinking of pairing the video card with either a E8600 or a Q9550 (or a Q9300 until Nehalem availability and price go down)