The P3-600 doesn't feature any big changes. It's based on the same .25 micron Katmai core as all the other P3 processors. It features 32KB of L1 cache running at the core clock speed and 512KB of external L2 cache running at 1/2 core clock speed. The processor is only available in the slot-1 compatible SECC2 form factor. Once Coppermine moves the L2 cache on-die, Intel will be able to manufacture the P3 in a 370-pin PPGA format. For now, the external L2 cache makes the slot-1 cartridge a necessity.
Basically, the P3-600 is a higher clocked version of the previous P3 processors. The only big difference we noticed was increased external L2 cache speed. To support the new 600MHz core clock speed, Intel had to up the cache speed to 300MHz. Of course, the processor still features all the SSE goodness that will let us through Intel's big blue door to the internet.
What is SSE anyway?
SSE stands for Streaming SIMD (Single Instruction Multiple Data) Extensions. SSE features 71 new SIMD instructions to improve floating point performance. Here's Kenn laying the smack down on in our original world exclusive Pentium III Review
SSE greatly increases the efficiency of floating point calculations by being able to work on four numbers in one clock cycle. 3DNow essentially does the same, but instead uses two instructions to handle half the load each. What makes SSE different is that it can simultaneously handle single-precision (for SIMD) and double-precision ("normal" x86 floating point) instructions - in short there is no penalty for switching from SIMD to normal x87 (in contrast to 3DNow!), as long as programs take advantage of the Pentium III's new processor mode.
That's right, programs have to be specifically coded to take advantage of SSE. This won't be a problem with new programs because they'll incorporate SSE optimizations as they're being written, but older programs will need to be rewritten to include SSE support. You can still benefit from SSE in non-SSE optimized games if your video card drivers are SSE optimized.