James,
All of the PRO series can run traditional Windows desktop applications in addition to metro/modern apps. The 64GB and 128GB (referring to the SSD storage space) models of SP2 both have 4GB of RAM, while the 256GB and 512GB models sport 8GB of RAM. The processor also jumps from the i5-4200u to the i5-4300u. I don't run PhotoShop, so I cannot directly answer your question, but it should all come down to how well PhotoShop can run on 8GB of RAM, and you probably have the experience to answer that yourself. I would NOT suggest the 4GB RAM models though for heavy-duty workloads. I bought the 256GB/8GB model, and I am extremely happy with its performance while running heavy-duty desktop executable (.exe) applications.

I haven't tried Photoshop personally, but you can install and run it since the Surface Pro has full-blown Windows 8 Pro and you can install any Windows application on it that you want. (Only the cheaper non-Pro Surfaces with Windows RT are limited to "Modern UI" apps from the Windows Store.) If you get the 256 GB or 512 GB Surface Pro 2 it will have 8 GB of RAM instead of 4 GB, and they all have fast flash memory for scratch file storage, so I expect Photoshop will run decently as long as you're not expecting workstation-class performance.

The artist for the Penny Arcade Internet comic strip has started to use a Surface Pro on occasion to draw the strip, and uses Adobe Creative Cloud programs on it. Search for "Penny Arcade Surface Pro" to get his take on it. (That was also for the older and slower first-gen Surface Pro.)

The Surface Pro 2 models, including the 256 GB/8 GB model, all have dual-core mobile Haswell i5 CPUs. The first ones shipped with Core i5-4200U processors, but around the December 2013 timeframe Microsoft started making them with faster Core i5-4300U processors instead for the same price. If you order a new one it will most likely come with the newer, faster CPU.