Not all PSUs are made equal. A good quality 500-550W PSU is perfectly acceptable for a simple desktop consisting of a quad core CPU, SSD, 8-16GB RAM, Geforce 660/760, since the total peak power consumption of this kind of combination will be no more than 250-300W (CPU 65-90W, GPU 130-170W, rest a few watts here and there), idle power consumption of a modern desktop is no more than 30-40W. This is all well within the limits of a good 500-550W PSU. A bad PSU, on the other hand, might have limited capacity on some of the rails or generally be less efficient. I recommend at least looking at the 80-Plus classification and understanding what the different white/bronze/silver/gold/platinum PSUs are capable of achieving.
The AI right now seems to be as much CPU as GPU bound. A mainstream quad core (i5) Haswell CPU is a sensible choice for that reason. 8-16GB RAM is advised (8 is enough for now, but a CPU will easily last 5-6 years and you will want to move up to 16GB at some point during that period, so get 8GB in 2 DIMMs leaving room for 2 to be added later). GPU sweet spot right now is Nvidia GTX660-660Ti-760 range of cards.
I bought myself a GTX760 to go with my 4 year old Core i7 / 8GB machine. Getting 120-140fps hotlapping at 1080p with max shadows, max world detail, low reflections (3 faces), 4x AF, 4x AA, 2x sparse grid AA. This drops to 60-70fps at the back of a 12-15 car grid, 100fps when racing the AI later in the race when they are spread out.