Your best option is to run the PSU externally, and I am doubting a full size GPU would fit. What is this PC going to be used for? If its a media center style then I would recommend a 5800K APU and 8GB DDR3 low profile. also use a 2.5 SSD and mount a custom DVD-RW. Its do able but def the PSU will have to be outside. I have seen pretty powerful PSU's like a standard Xbox 360 PSU being used on a ATX motherboard but would do some research on that.
7970 3GB *Unlocked* = 8 Threads
5770 1GB OCed = 2 Threads