windows benchmarks suggest that HD5770 = HD4870 = GTX 260 (with only a few percent difference in either direction, depending on the game)
Don't just calculate with the retail price though, power consumption is a factor as well. Here's some values:
Scenario #1: a life-time of 3 years with an average of 2 hours of load + 5 hours idle per day. (my scenario, since I often work at home and like gaming on weekends)
Scenario #2: a life-time of 3 years, 24/7 on, an average of 2 hours of load (for those warez kiddies
)
Calculating with a price of 0.20¢ for 1kW/h (I pay slightly more).
Code:
Costs for Power for different cards
Scenario #1 Scenario #2
card idle load kWh price kWh price
HD 5770 17W 108W 330 66€ 646 129€
HD 4870 55W 160W 652 130€ 1675 335€
GTX 260 38W 182W 607 121€ 1314 263€
So if you plan to actually use the card, the retail price is often secondary. And you'll quickly see that the 5770 will turn out to be the cheapest one - unless your power is paid by mom