Thanks - can you tell me what that means in terms of performance or functionality?
SSE4 brings with it a set of SIMD instructions. 3D API's such as Direct3D, future versions of Adobe Photoshop and several scientific applications keep close association with the SSE instruction sets and update each time a new set comes out. SSE in lay-man terms, 'speeds-up' math processing many fold. Game engines utilise processor instruction sets for their computation and their developers ensure they're up-to-date. For example, the Doom 3 engine could utilise SSE3 which was very new at the time when Doom 3 came out. The fact that you don't have a certain set doesn't mean the game engine won't work, just that it wont benefit from faster math processing. For example, Doom 3 on a AMD K7 will utilise only SSE and MMX for SIMD processing (because SSE2, SSE3 aren't there on K7).