Multi threading comes in to play when you talk about the
i7 series CPU's, they are all quad core but will show up as 8 cores thanks to multi threading (aka Hyper Threading). This has a big effect on video encoding and similar things (when converting from say DVD to AVI etc).
The i5 and i3 CPU's are not multi threaded but they are still lightening fast CPU's, personally I would recommend a bottom end i5 Sandy Bridge CPU and 4GB RAM will be plenty for what you want to do, a budget video card round about the £100-150 mark would be a superb extra too for CUDA support for those apps which support it though if you do not need that then you could look for something else.
Hope it helps
Jedi81.