Step 1 is always Intel or AMD
(Intel where known for their quality and performance, but recently have started to 'drop the ball'. So it's pretty even pros/cons for both)
Personally on your budget I would go for a high end Intel I5 or AMD equivalent
Step 2. Then get a list of motherboards which have a compatible socket.
I would be looking for 2 x SATA 6Gb/s ports (1 for an SDD and 1 for a HDD), at least 1600 DDR3 for ram, USB 3.0 for at least back ports etc
Check the locations of the PCI slots and the gaps between them (modern graphics cards have heatsinks/fans which are wide and can cover other PCI slots than the ones they are plugged into, or other components such has CPU heat sink)
Step 3 Graphics Card
You need to decided your requirements here..
Step 4 PSU
See what the recommended wattage is for your graphics card, go above this.
Get a quality brand with 5 years warranty (Such as corsair TX series)
Step 5 Everything else
Check the motherboards manual to make sure everything is compatible (these can be downloaded from the manufacturers website before you buy it)