Anand, ExtremeTech, among others, have posted a rather extensive article covering GeForce FX 5600 & 5200 Ultra performance. These are the mainstream models of the more powerful 5800 chip that has been available for pre-order for sometime now and has been shipping to customers for a couple of weeks.
For NVIDIA's bad, these 5800 cards are available in limited quantities and demand (I assume) hasn't been all that great compared to their previous products thanks to ATI who have been running the show for the last few months. Now NVIDIA is seeing their comeback opportunity in the mainstream, where most of the retail money is.
So, we now have ATI's released products (9700s & 9500s), NVIDIA's new FXs (5800s, 5600s & 5200s) and again, ATI's revamped line up (9800s, 9600s & 9200). Performance on ATI's new mid-priced cards still remain to be seen, though, better performance than current products is to be expected but not guaranteed.
What I can conclude for now is that if you are looking for a new videocard I would definitely recommend you to buy a Radeon 9500 Pro while they last (soon to be off the market); in simple terms it's a 9700 with a 128-bit memory interface. In the other hand NVIDIA new cards offer great value compared to their previous line up (we are hearing things about 5200 boards selling for less than $100) but still have a hard time keeping up with the competition.
You can get informed on the latest videocard prices here.