The Core i5 and i7 are good. You want a quad core processor. Intel uses a 4 digit numbering scheme where the first digit is the processor generation and the generations alternate between new designs and smaller lithography. A 'k' indicates an enthusiast processor which is typically faster. Here are some recent processors:
6xxx Skylake (14nm) was just released. The i7 6700 is fast and expensive. The i5 6600 looks like the best value for high performance. The 6500 should be better than most older cpus. The 6400 may not be faster than older Haswell chips.
5xxx Broadwell (14nm) is a die shrink of Haswell. These are rare because Intel had delays at 14nm. The 5775C and 5675C at 3.3/3.1 GHz appear slow, but they have a massive 128MB L4 cache which is very interesting for large data.
4xxx Haswell (22nm) is common. 4440/4460/4570/4590/4670/4690/4770 go from 3.1 to 3.5GHz. We have a 4570 and it's good. You should be able to get a good deal on these now.
These cpus are older, but still hold up well:
3xxx Ivy Bridge (22nm) is a die shrink of Sandy Bridge. The 3770k was a good performer.
2xxx Sandy Bridge (32nm) was a big leap over the previous generation. The 2600k was popular.
These cpus are showing their age:
Westmere (32nm, 2010) is a die shrink of Nehalem. Start of the Core i5's.
Nehalem (45nm, 2009) was the start of the Core i7's.
The AMD cpus were competitive on value but they have fallen behind. We have a Piledriver 8350-FX (32nm) and I thought it was a good value for 8 cores. It's slower than the Haswell cpus. AMD has a new architecture (Zen) in the pipeline which is worth watching.
As mentioned by acer, you want a 64-bit OS but I think that's standard. Windows is fine. Get lots of RAM to make your machine last. I would say 16GB minimum, 32GB is worthwhile. Get a solid state hard drive for your OS and programs no matter what.