For pure performance the HD4890 CrossFire is the fastest. If you're looking for the absolute fastest around 400$ this is it. Now if it's worth the extra $$$ it's entirely up to you. In my opinion, unless you're gaming on a 30" 25x16 monitor, it's just not worth it.
The HD4870x2 and GTX260 SLi are pretty much tied. I'd say this is prefferable. Obviously, if yo're running something lower than 19x12 or 19x10, those two are overkill too. But do not look at performance, when deciding between the two, because it's pretty much equal. Also, I would advise you to read up on the shortcomings of CF and SLi.
If you're running a 22" or lower (I hope it's not something lower than it!), just get a HD4890 and be done with it. Hell, it will probably have enough punch for 19x12 screens to run everything smooth.