Sun's sequel to Niagara, the appropriately named Niagara2, is going to be released before long (I hear sometime in April or June), so Sun gave a good talk on the chip and described it in some detail. Here's an information dump for the microprocessor geeks in the audience.
Niagara2 sports 8 SPARC cores, all of which are connected to 4MB of shared L2 cache by a large crossbar switch. Each core is capable of eight-way simultaneous multithreading, giving each Niagra2 chip a total of 64 simultaneous threads of execution. All of this thread-processing power enables Niagra2 to double its predecessor's throughput and performance/watt.
Sun also claims a 10X improvement in floating-point performance over Niagara.