If you're pressed for space and don't want to modify the card, go for a side mounted PCI fan bracket.

I use one to cool my Voodoo3. You can see a picture at the bottom of this post.

Is this blower even effective?

don't know if that fits, it's way more silent.
download/file.php?id=18100&mode=view

The Voodoo isn't a overly hot card, I think as long as it's getting some airflow over it even if not much things are greatly improved.
I used to hang a small fan off the back of the card using cable tie's into the hole in the corner of the card.

(Different machine but same idea)

My problem is, that i want to run it with a p3 750 i tried it runs without any issues, but i do not know how the heat will be if i run it bit longer

Realistically you should be fine but I agree with your thinking and would run something over the card just to be on the safe side