The Warp3D driver that is under development will be for the Radeon HD 5xxx-6xxx range (excluding the Radeon HD 69xx series, as those are too different). Exactly which card depends on what you want, but I strongly recommend getting a mid to high-end card such as a Radeon HD 6670. The Radeon HD 6670 is noticeably faster than the 6570, and I have one that fits into a single slot: Sapphire Radeon HD 6670 Low Profile (11192-18-40G).
Hans