Im running 1 league with a large database. Here is my spec:
1.8GHz dual core processor
2 gb ram
120 gb hard drive

How many nations can i safely load all players from without my game being slow?

I have a much worse computer than that, but I genrally don't put on any, or maybe one small country whose national team I want to manage. Depends what you define as slow, and on what nations your loading players from. I'm no computers expert (and I'm sure someone on here is) but I wouldn't try more than 3 or 4.

I have similar specs to you and ran 7 with 15 leagues, that seemed perfect for me.