NVIDIA's 9800 GTX+ announcement came right in time for the AMD HD 4850 launch, to slow down its momentum. The move from a 65 nm to a 55 nm GPU design does offer some benefits. First of all the production price of the GPU goes down. This means that the card could be offered cheaper than the original 9800 GTX. Second, the heat output under load is reduced as well, allowing higher clocks while staying in the same thermal and power budget.
While the GeForce 9800 GTX+ is faster on paper than the HD 4850, I seriously doubt you will feel the difference while gaming. Considering other metrics like price/performance and performance/watt, both the normal GTX+ and the GTX+ Amp! lose against AMD's HD 4850 offering. One advantage the GTX+ cards have, is that they have more overclocking headroom left than ATI's offerings. Zotac's AMP! Edition is a bit too expensive for the performance added. If you are looking for an easy solution that is faster and does not require tweaking, then the Zotac 9800 GTX+ Amp! Edition is for you.