One thing you might consider is if you're planning on playing any games with physx, you should probably go with the GTX 560 ti as opposed to the 6870 since you don't have a very powerful processor to offload the physx to. Some people say it is a gimmick and its not supported by every game but for the games that do use it, I find it to be a nice extra bit of eye-candy.
Also, I think the only thing the HD 6870 beats the 560ti at is power consumption (non OC'ed 150 watts for the HD 6870 and 170 watts for the 560ti).
That said, if there is any way you can step it up to a 660 or even a 660ti I would go that route as even the 660 is considerably faster than both the 560ti and the HD 6870. Here in the states, the 660 is only going for about $20-30 more than the last gen 560ti.