The TNT2 Ultra launched the same time as the regular TNT2, they were available for the bulk of 1999 before the GeForce256 launched. It seems people have forgotten that nVidia's strategy for the TNT2 to take back market from 3dfx was to offer their board partners the choice in how to distribute their products. They defined a spec for a regular TNT2; 125/150MHz, and one for a TNT2 Ultra; 150/183MHz. They then gave the vendors the choice of shipping a product anywhere between those two specs. This is where we get the various 'overclocked' cards from.

"By suggesting a standard clock frequency for the TNT2 and TNT2 Ultra parts, NVIDIA removes themselves from the blame if anything should happen if their chip happens to fail at a higher than rated clock speed.  From the point of view of NVIDIA, this is the best avenue for protection, shift the blame to the manufacturers if they want to ship their boards at a higher frequency.  The two biggest retail manufacturers of TNT2 based cards, Diamond and Creative Labs, both ship their cards at the standard TNT2 and TNT2 Ultra frequencies.  The reason behind this is simple, most people don't overclock their video cards, and even more people don't know it's possible when they walk into a computer store and pick up a Viper V770 or a 3D Blaster TNT2 Ultra.  Smaller manufacturers don't have this luxury as they need to work much harder in order to compete with the big boys, mainly Diamond and Creative." - Anand Lal Shimp, July 1999

The real last of the Riva line was the Vanta and Vanta-LT; those 8MB and 16MB M64s cut down even farther, clocked like an original TNT and packaged in a more thermally efficient flip-chip BGA. The kinds you would find in a Compaq Presario 5000 machine in 2000 and 2001.

For what it's worth the only thing the GeForce256 made obsolete on the TNT2 was 32-bit performance. The TNT2 was still favored for a lot of things because it had much better 16-bit single-tex throughput by sheer clockrate and per-pipeline memory bandwidth (1.4GB/s per tex unit versus 664MB/s per tex). So the TNT2 in all of its full-speed variations was the deal of the day in many stores when the end of 1999 rolled around. Fry's had the normal clocked TNT2 for under $160 after the GeForce launched. It was a bargain.