Sweet spot for power vs price seems to be an i5 CPU right now. Especially since you probably don't really need hyperthreading.
I have had good luck with both Asus and Gigabyte motherboards.
Nvida cards are overpriced right now. ATI video cards have much better price to performance ratios. Nvidia has somewhat faster driver updates, and due to their aid (bribery) to game companies, they sometimes have things tuned better their way. Certain video and rendering software packages can really make use of Nvidia's architecture better, but those are fairly specialized options.
Cases are really a personal decision, get what you like. Just check out reviews to make sure they don't have any hidden gotchas. I like big airy cases, as I replace parts with some frequency and like the working space.
On hard drives, if you can manage it, get an SSD for your OS drive, and a big mechanical WD drive for you main storage. Be sure and check on the hard drive warrenty: Usually the longer the warrenty, the higher the construction quality. A shocker! Probably a WD black HD. I generally prefer SSDs from Samsung or IBM, but others can be good. I would not get an OCZ one.
If you listen to a lot of music on your computer, you might consider getting a sound card, but you can otherwise really get by with motherboard sound now. I have an old Soundblaster still though.
Good luck with it!
".... I have always found that plans are useless, but planning is indispensable." - Dwight D Eisenhower