All are important. Photoshop stresses every part of your computer.
Whenever trying to figure out what combinations of hardware give good performance I start by looking at the Passmark benchmark results.
Their web-aware Performance Test application, which you can download and use for a while for free, facilitates people around the world testing their systems and uploading the results, then you can use it to download baselines from different systems and compare graphically.
Notably systems built around the i7-3930K Sandy Bridge processor are topping their all-around score performance chart at the moment.
Some general guidelines for use with Photoshop I'd suggest:
- Get at least 16GB of RAM, more if future-proofing is important to you and you can afford it (I have 48GB). I strongly recommend ECC.
- Get a video card that scores at least 1000 on the Passmark G3D test (pretty much any card at $100 and above does that nowadays). You don't need God's own high-end card for Photoshop, but don't skimp either.
- Consider SSD storage, as SSDs have arrived in the mainstream and no longer need a lot of special care (though they're still expensive but dropping).
Only a few months ago I embarked on the same search you're doing. I kind of normally follow a kind of off-beat track myself... I watch for off-cutting-edge workstations to become available (e.g., off lease) on eBay, then outfit them with great hardware components, such as an array of SSDs and a mid- to high-end gamer video card. I like these kinds of systems because they're optimized for high throughput and reliability overall vs. raw speed at specific things. There's nothing like having dual Xeons with tons of cores and hyperthreading for 3D rendering. Right now I have a Dell Precision T5500 system that has top-of-the-line parts in it. This config cost over $10K at Dell just before they released their T5600 line, then dropped WAY down after. Per the Passmark benchmark it's still the most powerful Dell as yet benchmarked based on overall score.
Thanks! A more complete response than I hoped for. I will do my research using your response as a guide. I am in the akward positon of using a Mac at my home studio and a pc at my photography studio. I may just end up with another Mac, but honestly, I find some of the 3D software I use is not as bug free for the Mac as it is for PCs. Thanks again.