Hmm.. I've never messing with a 9400 GT before. Probably the best thing you can do is test it out a little. Monitor your temps, and just mess with one clock at a time, say:
1. step your Core up 5 Mhz, and run a test (a benchmark with 3dmark 06 or something like that) and keep repeating that process until you start seeing artifacts or crashes or whatever (or your temps get higher than you like) and then step it back 5 or 10 MHz.
2. Repeat step 1 with Memory
3. Repeat step 1 with Shader.
See where you end up. It depends on your individual card to some extent and how friendly it is. I would imagine your PSU should be enough for it. Do some testing and let us know where you end up!
Good luck and let us know if you have more questions🙂