You are the first I've seen on this forum that tried Leopard with 5770/5870s.
And while a 4,1 with 10.6.3 or .4 could boot and get video, it was 10.6.5+ and not the original .4 that worked.
So I'd say $30 for 10.6 DVD or a pack price for 5 probably.
Lion has support for one or two PC 6xxxx series cards natively it seems but that is risky path.
Everything I have seen and read, yes, you need 10.6.8.