EPTPreface:
Starcraft 2 is optimized for 2 CPU cores. Any more cores will not see a significant performance advantage.
However, many newer CPUs have built-in load balancing features which may enable the CPU to offload work onto other cores or threads, so you might gain a small boost with those CPUs.
Benchmarks:
http://www.pcgameshardware.com/aid,766589/Starcraft-2-CPU-benchmarks-Intel-on-top-quads-without-performance-benefit/Practice/
http://www.bit-tech.net/hardware/cpus/2010/08/18/how-many-cpu-cores-does-starcraft-2-use/2
http://www.legionhardware.com/articles_pages/starcraft_ii_wings_of_liberty_beta_performance,6.html
http://www.techspot.com/review/305-starcraft2-performance/page13.html
http://www.pcper.com/article.php?aid=958
To summarize the benchmarks, the faster your CPU operates as far as raw performance, the faster the game will go. As many reviewers term it, the game has a linear performance scale.
Intel CPUs:
Core 2 Duo/Core 2 Quad processors will still run the game fairly well, just as they run any other game and applications fairly well to this day. The reason being the majority have high clock speeds, in and around the 3ghz range. Earlier C2D/C2Q processors had speeds in the range of 2ghz and sometimes lower, and will not run the game as fast. If you have these processors, overclocking them to higher speeds will see a significant performance boost to match those of later era C2D/C2Q chips.
Intel Core i7 processors see significant benefits from architectural improvements. This is why a Core i3/i5 will run the game as fast, if not faster, than a higher speed Core 2 chip.
AMD CPUs:
AMD's Phenom II and Athlon II CPUs are very similar and run at similar GHZ speeds. There are differences in processor cache which benefits some applications, but not SC2. This is why the majority of AMD's chips run the game at around the same speed. Going back to SC2's linear scaling, the faster you run your AMD chip, the faster the game will go.
What should I Buy?
This is a question with no set answer. Strictly speaking, you can build an extremely cheap computer around a lowly AMD Athlon II processor and the game will run fairly well. But there are many other applications and games that you will probably be playing now and in the future that will benefit from faster GHZ speed and more cores.
For example, a lot of you are trying streaming such - this requires a very fast processor for high quality, and video encoding benefits immensely from multiple cores, thus a newer 6 core Phenom or 8-thread Core i7 will be the best choice. Of course, these are also very expensive CPUs.
On Video Cards
SC2, as it turns out, is fairly CPU bound. Meaning, less expensive video cards like a GTX 460 will run the game on Ultra at high FPS, reaching speeds within reach of much more expensive cards like the GTX 480 or Radeon 5870. But again, SC2 might not be the only game you're playing, so make sure to do research with your favorite games to find the best card for yourself. You can safely keep in mind that most modern video cards will run SC2 fairly fast.
Recommendation
As per building a new system, strictly speaking for SC2, I would start with a competent video card baseline, the GTX 460, and then build a system around that, adding in components as your budget allows. I am not saying to always opt for the GTX 460, as I mentioned above, make sure to do your research and find the best card for all your games, but the GTX 460's price gives it excellent bang-for-buck and should be considered the baseline for purchasing. If your budget simply cannot allow for that, the next best bang-for-buck would be the Radeon 5770.
As per CPU, if you don't plan on doing any kind of streaming or audio/video work, then a 2 core processor is more than enough. These include any AMDs with "X2", and Intel Core i3 & i5-5xx/6xx processors. Otherwise, AMD X4, X6, and Core i7 processors will help alot with your streaming.
Finally ...
As of this time of writing, AMD will be releasing new video cards in a couple of weeks. I have no idea as to their performance or pricing, but if you don't plan on purchasing right now, it would be better to hold off until these cards come out to have a broader range of options. There might even be price drops on currents cards, so look out for that.
Also, Intel will be releasing new CPUs come January, which are fairly fast ( http://www.anandtech.com/show/3871/the-sandy-bridge-preview-three-wins-in-a-row/11 ), and are rumored to be "price competitive" as well. Just another thing to think about.
If anyone has further suggestions or comments, or something I missed, please speak up! It will benefit all of us.
