There is no such thing as a 'good' 9400gt
They are all to slow for gaming (half the speed of a 8600gt/9500gt) and outclassed by the low end ATI 4xxx series for media box usage.
As for your other questions, though.
Yes, you can use any PCIe 2.0 card in a PCIe 1.x slot.
No, the 9400gt does not require an additional power plug.
A PCIe 1.x slot can deliver 75w and it only requires 50w peak.