Your PSU can handle that card(and better ones) easily but I believe you would be best off sticking with a 2.0 card on a 1.0a slot unless you can easily return the card if it has problems. 2.1 cards sometimes have issues with 1.0 slots but not always and you wont know until you try. If there is a recent BIOS update for your motherboard it may actually handle the issue and then you don't need to worry.
I Would Chosse A Better PSU But You May Be Ok, And Id Try It In The PCI-E Slot First If It Works Then Your Good, I Dont See Why It Wouldnt. And You Will Not See Any Performance Problems At All Using A 1.0 Slot