Read this thorough review on most of the Workstation/Professional cards.http://www.cgchannel.com/2011/10/review-professional-gpus-nvidia-vs-amd-2011/
In the review, it shows a V4800. But, the V4900 is slightly higher clocked, and performs slightly higher, nothing significant over the V4800.
Actually, according to standard benchmarking, the 4900 is fully 17% faster than the 4800.http://www.videocardbenchmark.net/video_lookup.php?gpu=FirePro+V4900
I realize that this doesn't perfectly reflect workstation graphics performance (which these cards are tuned for) but it's a good way to compare cards.
Just thought I'd let you know. Currently the 4900 and the 4800 are very close in price (on Amazon) so go for the 4900 if you're choosing between those two.
One thing to note is that the Quadro 2000 is rated as 25% slower than the 4900, in the above benchmark. Again, this doesn't reflect performance in a production environment, which is highly dependent on drivers and optimization, but it's something to keep in mind.