The GTX 480 is now considered the single fastest GPU, but because of its high heat output, high power usage and higher price compared to its ATI counterparts its not the best value.
Yep, any high OC'ed card or refresh will be an improvement like the 4890 over the 4870. The 4870X2 was still better then too. The next hope for a major improvement won't be until end of 2010/beginning of 2011.