Since I've added an alpha channel to my texture, something very weird (perhaps drawing order related?) is going on in my viewports (all of them, not just perspective), like this:

It software renders fine, and it goes back to normal if I go back to my texture with no alpha. Does anyone know what's going on? It looks really freaky!

I'm using Version 6 (PLE), comp specs: 2GHz, 512MB RAM, Radeon 7000 graphics card.