If the PSU is the same one as in this xbit review of the case http://www.xbitlabs.com/articles/other/display/chenbro_6.html
, then you'll be fine.
The review unit came with a Fortron Source PSU. May even be good enough to power you when you get your 2nd GPU. Though at 300w, that's pushing it a bit... if it was 350 however...
20 pin/24 pin - doesn't matter much. You can get an adapter either way if you need to.