What is a good fps for a p90 airsoft gun?

Depends on what you will use it for. If you're using it for CQB skirmishes, 300-350 fps is good enough (of course with appropriate weapon i.e P90, MP5, MP7). If you're using it for middle range urban/woodland skirmishes (i.e. with M4/M16 family, G36, SCAR, HK416), you can go for 400-450 fps. If you're using it for sniping (Bar-10, M40), you can go up to 600 fps.