PCI-e is going to fit in any motherboard except mini ITX because that only has one PCI-e slot and you'll want a graphics card too.
In terms of the CPU I think an i5 would be best, excellent gaming performance and still very good for your Photoshop etc.
In terms of RAM, I'd get 8GB but on a motherboard that can accept 16GB with 4 slots so you can easily upgrade it if you need to.
If you can make do with 250GB as well I'd get a SSD now and perhaps add a hard drive later, it would make the whole system feel faster and it is much easier to do it this way around rather than trying to add the SSD later.
PCPartPicker part list:
http://pcpartpicker.com/p/DwsChM
Price breakdown by merchant: http://pcpartpicker.com/p/DwsChM/by_merchant/
CPU: Intel Core i5-4460 3.2GHz Quad-Core Processor ($176.95 @ SuperBiiz)
Motherboard: ASRock H97M Anniversary Micro ATX LGA1150 Motherboard ($67.89 @ OutletPC)
Memory: Kingston 8GB (2 x 4GB) DDR3-1600 Memory ($38.99 @ SuperBiiz)
Storage: Crucial BX100 250GB 2.5" Solid State Drive ($83.99 @ SuperBiiz)
Video Card: MSI Radeon R7 370 2GB Video Card ($149.98 @ SuperBiiz)
Case: Fractal Design Core 1000 USB 3.0 MicroATX Mid Tower Case ($34.99 @ Directron)
Power Supply: Antec EarthWatts Green 380W 80+ Bronze Certified ATX Power Supply ($38.99 @ SuperBiiz)
Total: $591.78
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-08-16 06:49 EDT-0400
Very similar to the build suggested above to be honest but should do a good job for your usage.