playfulness or aggression toward the kitties

My 15 month old Aussie/Border mix terrorizes the cats. We have them separated by a toddler gate at the bottom of the stairs - the dog lives downstairs and the cats live upstairs. The cats enjoy sitting on the stairs looking down into the living room but the pup often scares them away. But there ae some times when she just looks at them without showing too much interest - but the gate is always separating them. When she notices the cats are on the steps, she tends to bounce toward the stars while whining and sometimes barking. She also will lie down with her front paws flat in front of her and her butt in the air - tail wagging and whining when she sees the cats on the stairs. On the rare occassion that one of the cats will venture into the downstairs, Bella usually gets crazy excited. She will chase the cat but usually stops when the cat tries to scratch her or runs away. She tries to get close to the cat but the cat hisses and swats at her. Bella has gotten scratched on more than one occassion but it doesn't seem to stop her.
I don't think they will ever be "friends" but I would like for the cats to be able to come downstairs and spend time with us without being afraid. And I'm not sure how to tel if Bella wants to play with or eat the cats!
She seems to have quite a high prey drive - she will nearly choke herself trying to get to the squirrels and birds in the yard. She has even cracked a window trying to get outside when a squirrel was playing on the porch! There have been a few bird/squirrel victims already and I'm worried if that's what she wants to get closer to the cats for.
Can anyone tell me how to tell if she is being playful or aggressive toward the cats?
Thanks.