It might be clear to just get the fastest available, but I have a couple of questions.
In earlier Renoise versions it seemed to be better to turn of hyperthreading.
is this still the case for Renoise 2.5?
is this still the case with new Intel core I5/I7 CPU’s?
And which CPU is likely to be the best choice for the future development of renoise, considering the 64 bit era and so on.
Didn’t think the i5 had the Hyper Threading of the i7 (or is that the i3?)
Hyper Threading on the new i7 has nothing to do with Hyper Threading on the old Pentium 4. Not seen any tests with Renoise but have with a lot of other DAWs such as Steinberg. On these P4 HT was worse when turned on but i7 HT was a lot better when on.
Does it actually work with Renoise now? 2.0 didn’t even recognize HT cores.
Have you got an i7 CPU?
Unless you’re doing something really intensive, Renoise seems to cope well with most modern CPUs. My Macbook has a Core 2 Duo, 2.2ghz and that runs fine. My PC has an i5 750 (2.67ghz quad core, no hyperthreading) and it is LOVELY.
Or you could try this… http://ark.intel.com/Product.aspx?id=47932 
does it matter, in terms of performance if you run the 32 bit Renoise on a 32 or 64 bit OS?
Shouldn’t make a difference - Use the version that suits however much ram you have. A decent amount (4gb) will require the use of a 64-bit OS for you to actually gain access to most of it. And if you don’t, going 64 bit now will give you less hassle if you upgrade your RAM in the future.
but it still seems to happily use over half of the virtual cores when rendering?
Renoise + i7 = win 