the chipset supports it? I was told on another forum that at most I can put on a gt 630 \ 620
The chipset is not an issue. And your 300w PSU should be fine. (Although they call for a 350w PSU, so you won't have any headroom for loading down the PSU with other power-hungry add-on's in the other slots.)
If you want to be sure, find the 12v rail(s) current rating on your PSU.