I'll agree with Ludi and DPete27. Don't spend money on any gaming graphics card slower than a Radeon HD6850.
I'm a proponent of the trickle-down theory of computer upgrades. Your GeForce GTX460 is still capable enough, so upgrading your graphics card and letting the GeForce GTX460 trickle down to your sister is a good plan. You get an upgrade and your sister gets an upgrade, too. Win-win.
i7-4770K, H70, Gryphon Z87, 16 GiB, HD7950, SSD, 2 HD, Blu-ray, SB ZX, TJ08-E, SS-660XP², 3007WFP+2001FP, RK-9000BR, MX518
i5-3570K, GeminII-S524, P8Z77-M Pro, 16 GiB, HD7770, SSD, 2 HD, Blu-ray, InifiniTV4, NSK2480, 55" TV; Asus UX32VD