I am afraid that I am not an electronics engineer, just a humble grease monkey. As I (sort of) understand things, there are two "channels" in your PSU and one may not be functioning properly. I hope that one of our experts will step in and explain it properly

The Duck has an interesting point. Normally all onboard stuff is enabled by default, BUT mistakes do happen so please check that the motherboard jumpers are set correctly as shown in the manual/instruction sheet.

Yes, a 300W should do, but only if it is working properly and is actually giving you the performance (it is good quality). Personally, I would still have gone for 400W+

You might test it by disconnecting the CD/DVD, floppy, speakers and so forth, which will reduce the power demand?

Unfortunately, I have heard of a few defective ABIT boards recently, so Ghost~ has a good point as well. In fact, I was drinking with a friend on Friday evening and he told me that it was only the third ABIT MoBo he received, that worked! that is not good news as it meant that he had effectively built three machines, but could only charge the customer for one!

That said, even if it is a defective MoBo, I would still get a better PSU