ok a 250 w psu is not enough... you could spend 20-50 bucks at newegg and get a 400+ psu that would give you something to work with you cant put enough hardware in that system to expect decent playback . Or decent resolution on any movie!
It is a mATX case and normal sized PSU's will not fit. AFAIK mATX PSU's max out at 300w, which I have found online and may upgrade to a 300w just to be safe.
So the system must work with a 300w PSU