i5 quad cores don't have hyperthreading - you only get 4 processing threads. But the dual core i5's have hyperthreading, so you can also get 4 threads with those, too. All the i7's have hyperthreading.
As far as building an audio computer, I used a quad core i5 as the primary muscle in my DAW workstation for a few years without any problems at all. I work mostly in the box using synths and sample libraries, and a core i5 wasn't a barrier to getting stuff done. I had to be careful how I worked, sometimes -- I was very judicious about freezing synths, not overloading projects with needless plugins, etc -- but I was fine with an i5.
Take that with a grain of salt, of course, as that's just my own experience, based on my own workflow habits, etc, so your mileage may vary. But I personally think a well-spec'd i5 based system can make a perfectly usable DAW work station, especially if paired with a soundcard with solid drivers. Having good audio hardware and driver support is probably the most essential factor of all, I would think. I'd rather have a moderately powered i5 system with, say, RME interface and drivers, than a beast of an i7 system with dodgy audio drivers and hardware.
I eventually moved to a quad core i7 -- and I just upgraded again to a 6 core i7 -- and I will admit the difference is marked, especially in terms of how many synths I can run, and how fast my system will bounce tracks and similar. But if I was working on a tight budget and building from scratch, I'd have no worries about going with an i5. Just make sure you get decent amount of quality brand memory, the best power supply you can afford, and solid audio hardware. So, if it was me I'd put 100 extra GBP into those elements, before I'd put it into an i7.
Just my opinion, of course. I'm curious to hear what others think...