Which upgrade path to go?

current system:

Cpu: q6600 @ 3.0 ghz
mobo: Evga 650i ultra
PSU: Corsair tx650w
GPU: evga gtx 260 core 216
case: haf 932
hdd: wd 160gb
cooler: corsair h50
soundcard: asus xonar dx

Should I find a way to sell my gtx 260 to get a gtx 285 or hd 5850? or should i stick with it and have a new mobo (suggest a mobo <200) and get another 260 to go sli? (don't mind the psu)

and also, I'd like to upgrade my hdd, 1 tb maybe. what is a good one? I've seen that the f3 performs best on price/performance.

I've ruled out my choices:

1st, if i will go with sli on gtx 260, I'll be spending
140 for evga 750i sli
190 for another gtx 260 216
that's 350 all in all

2nd, if I will go for 5870 (should i buy a newer mobo?)
400 it will cost

that's just $50 difference. but if i will buy a newer mobo, it will add more price to the 5870. is it worth going with second choice? or will 2 gtx 260s beat 5870?

5850, no other choice