660ti is a great card no matter, But since ull be playing at 1080p with AA and MSAA then u shud see this..
U can see very clearly that compared to 670, 660ti begins to fall 15% behind once u crank up AA and MSAA.
Now regarding ur choice between 7950 and 660ti.. Id say get 7950. Given that there r sooo many mixed opinions regarding 7950 and 660ti. Check this out
This page predicts the average performance gap between 7950 and 660ti taking into consideration around 10-12 different reviews...
stock vs stock, 7950 is 3-5% faster than 660ti.
7950 OC is around 8-10% faster than 660ti OC. ( the reason being 7950 scales excellent when overclocked )
In addition 7950 overclocking potential is tremendous compared to 660ti with additional 3gb of Vram..
Following is the comparison between Highly overclocked 7950 and 660ti.
The results r pretty clear here as well.
SO in the end, if u are on a very tight budget, cant spare a penny extra then overclocked 660ti looks good.
On a slightly flexible budget u can add 20 dollars more to get overclocked 7950..
Or if u have more cash then u can get Gtx670 for 400 dollars .This card is also overclocked and performs 5-7% faster than overclocked 7950. ( but note that upon manual overclocking, 7950 is around only 1-2% slower than maximally overclocked 670 for 50 dollars less. )
SO the choice is pretty simple, It depends upon ur budget, 7950 falling right in the middle between 660ti and 670 both performance and price wise.. But if i were u, Ill go for 7950 because of extra Vram, high bandwidth, huge overclocking headroom and more future proofing..