I believe these are the specs for the AMD Radeon R9 270X...
http://www.amd.com/en-us/products/graphics/desktop/r9
Its pretty guaranteed that you will have to upgrade your power supply no matter what though since the 400w really can't support many options. I ran my new card with the default 400w for a couple months until I found the psu I wanted.
Also, I am unsure as to how true this is though, but I was told if you have a Intel CPU, you should stick with Nvidia graphics and AMD/ATI stick together.
For the ASUS Essentio CG8350...
Graphic Options (Listed on ASUS website):
NVIDIA GTX560 Ti (1G/D5)
NVIDIA GTX550 Ti (1G/D5)
NVIDIA GTX460 DX 11
NVIDIA GTS450 (1G/D5)
NVIDIA GT530 (2G/D3)
NVIDIA GT530 (1G/D3)
And in case you...