Is my Radeon 6870 still any good for this?
I might give middlecoin a go if it cashes it for me.
It should get you 300~330kh/s. As a comparison I got 510kh/s from my 6950 unlocked to 6970 and now getting 1475kh/s from my crossfire R9 280X.
My current performance is getting me on average $15 a day. So by my calcs your card would only get you about $3 a day. You decide if that's worth it.