Ultimately software needs to be written multi-threaded to use the cores and get the full performance out of the machine. The applications you mention, Photoshop and most 3D games are already written to use the hardware and have been for years.
Just don't expect notepad or ms-paint to be any quicker :-)
Multi-cores are going to be ubiquitous in the future, (if they aren't already), and Microsoft and Apple are working very hard to make using all the cores as easy as possible for developers.
I personally don't look at the number of cores, but I look for the sweet price / performance spot when it comes to processors.
Normally Intel have their high end processor with their highest clock speed, and number of cores that is really expensive. I then look at the lower clock speed versions of that processor and find that there is a reasonably priced high end processor. Not the highest end processor, but still a really good one for a good price.
I then take the money I would have spent on the processor and use it to put as much memory in the machine as it will take / I can afford.
I think that gives the best performance for price, aka bang for your buck.
@SashaChedygov Even when they aren't used by the game additional cores can help by handling the overhead of other apps running in the background. Testing with World in Conflict Toms saw a 20% FPS penalty from a background AVG scan on the quad, and a 60% hit on the dual core system. Without the virus scan running the FPS difference was negligable. http://www.tomshardware.com/reviews/cpu-cores-performance,2373-10.html
– Dan is Fiddling by Firelight – 2012-07-26T14:43:16.813@DanNeely: True, but I don't think most people run CPU-intensive apps in the background while gaming the majority of the time. If you do, then a quad-core is probably worth it, but that's something that should be looked at on a case-by-case basis. That said, I just upgraded to a quad-core machine and am glad I did. – Sasha Chedygov – 2012-07-26T20:37:56.663
@SashaChedygov Assuming the average user, to include casual gamers, has the technical literacy to make sure their virus scanner/etc are configured to only run in the middle of the night is naive. Windows update defaults to running on boot and since it can strangle a single core box, I suspect it would cause problems on a dual core box if a game with 2x heavy threads was running. Then there are times when a page in your web browser barfs and starts running an infinite loop in javascript. Without checking task manager this isn't always easy to notice since the browser UI doesn't always hang. – Dan is Fiddling by Firelight – 2012-07-26T20:47:05.620
3"most 3D games are already written to use the hardware" -- You'd actually be surprised at how few games take advantage of multi-core processors. Those that do generally only use 2, so having a quad core won't help in that situation. Though I imagine the majority of future games will be written with multi-core in mind, it's not the case now. – Sasha Chedygov – 2010-01-04T23:20:55.577