4890 - best DX9 budget card. 4870 - 2nd best budget card.
Have to correct you on that, those cards are not DX9, they are DX10.1. Also I don't think they fit the bill of a "budget" card, even with the 4870 you're still spending about $150 to get the 1GB version.
Back on topic....the 4890 will walk all over the GTX260, so if they're the same price then definitely buy the 4890