Make sure you check your psu amps to see if they meet the amps of the ps3, because if you start gaming and your pc psu cant handle it, it might kill the ps3
Edit: from an article, the ps3 psu is rated at 380W. I read that a pc psu to work needs at least 400W with 30A on one 12v rail.
If your 250W psu is a good quality one, you can try it. If not, try to use one that is 400W and 80+ rated
Check this for your amps research: http://www.ps3devwiki.com/wiki/Power_Supply
You can guide yourself in here:
Playstation 3 - ATX Power Supply Hookup - YouTube