HD4850 and save the rest of the money for a future system.
At that resolution a GTX260 and HD4870 whilst offering better outright performance (in terms of FPS) won't actually offer anything more in terms of playability (the HD4850 will already offer more than playable framerates).
^^^This, at the resolution you'd be playing at you wouldn't anymore than a 4850. Take the money you would have spent on the gtx295, get a 4850 and with the change upgrade the rest of the system.