From GTX 460 768 to radeon hd 6870

It would be around 25-30% faster.

Yeah with that resolution you won't see dramatic increases..but, the 6870 is slightly more powerful than the gtx 460..30%-35%..maybe more in some instances is the boost your looking at... If you can get a good deal for them go for it..
Take a look at the comparisons
http://www.anandtech.com/bench/Product/315?vs=290

In a lot of cases you get some pretty good gains!...but, if your looking for longevity go for a 6950...down the road if you look to crossfire, the 68** series may be hard to acquire and it's a bit more beneficial to gran the 6950's if your looking for some "future proof-ness"

At 1440x900, no. If you have low frame rates then you should look into upgrading your CPU instead.

most of my 2009-2011 games got minimum of 55-60fps all in ultra high settings.
except for dragon age 2 only high settings got minmum of 45fps in some place, never tried metro2033
i never use 8xaa above!

so whats wrong with minimum 45fps? maybe upgrading your monitor should be on the table. a nice 22" monitor is about ths eame price as a 6870. If your motherboard supports sli, get another gtx460. what cpu you running?

its only dragon age 2 that have minimum 45fps @ high settings the rest of my games got 55-60fps ultra high settings, 22"hd monitor here cost about 170$ samsung brand...
6870 cost 240$ powercolor brand.

i need to upgrade my gfx to 1gb if i ill use 1080p resulotion my gfx has only 768mb...
so i really need to upgrade my gfx fisrt before getting 22"hd lcd monitor..

btw im using phenom II x4 955be processor...

get a 6870, you won't regret it, you can pick them up for like 170$ now, crossfire is really good too, they beat a 580gtx

If you are overclocking the GTX 460, you won't see much of an improvement.

That 460 is a good card especially for the resolution you are running however if you were to get a 1080P monitor I would maybe grab another 460 or upgrade.