Ouch! That is a very low capacity power supply. I guess that I shouldn't be surprised. A slimline system is designed to be low power, low noise, low profile, perhaps primarily, low cost. If you read the user comments at newegg about the KFA2 GeForce 8400 card, they say that Nvidia recommends a 350W power supply on the box. I have not been able to confirm this, but note the comments from 2 users who had 280W and 275W PSUs and said they got it working. They knew that they were below the recommended power.
Another power draw of concern is the CPU which HP lists as an Athlon64 X2 with a TDP of 65W. That's quite a bit of power gone right there, so any graphics adapter would have to be a very low power design to work in this system. I'm not sure there is such a card, but I'm pretty certain that the KFA2 8400 is not going to work.