The Athlon 4 gets SSE
If you'll remember back to the release of the original Athlon microprocessor, it received a total of 19 new instructions that AMD dubbed Enhanced 3DNow! These were all Single Instruction Multiple Data (SIMD) instructions and they actually turned out to be a partial SSE implementation. The Athlon 4 takes this one step further and adds 52 new instructions for what AMD calls 3DNow! Professional which is really the same SSE instructions that the Pentium III debuted with over two years ago.
While the SSE optimizations won't buy the Athlon 4 an incredible amount of performance in most real world situations, they will ensure for more equal treatment under certain applications that were arguably more SSE optimized than they were 3DNow! optimized.
For those of you that aren't familiar with SIMD instructions, they allow a single instruction to be applied to multiple datasets. This comes in handy with many 3D operations such as the transformation of coordinates from mathematical space to 3D vector space. These transformations require quite a bit of repetitive math, including multiple addition, subtraction, multiplication and division functions. By applying SIMD principles to this transformation a single add command can be applied to multiple data points simultaneously saving quite a bit of time.
AMD's x86-64 line of processors will actually receive a full set of SSE2 instructions, but for now the Athlon 4 is only compatible with the original SSE instructions. Don't expect performance to improve too much because of this, but maybe it will quiet those that always accuse benchmarks of being "SSE optimized."