Hyperthreading yields extremely noticeable benefits in programs that are able to use multiple cores. Most of Adobe's CS software for example...
In games you won't notice it, but in professional software you sure will.
When running programs like Adobe CS4, the Video Card may affect the Hyper-threading of the i5 processor. If the video card has OpenCL, CUDA, and most specially openGL technology, the processor may not be 100% available because the GPU of the video Card is the one the program is using. Thus the Hyper-threading of the Processor won't be that noticeable.
Or I may just be wrong. I'm no expert but I think this theory sounds right.