I'm sorry to all of the NVidia lovers out there, I'm not biased toward one or the other when it comes to GPUs, I run an NVidia card currently myself, but honestly, the HD7870 is a better card all around than the GTX660 (The OP seem's a bit confused as he says GTX600 non-Ti in the title of the thread, but then goes on to write "GTX660Ti-Boost", which I've never even heard of [there IS a GTX650Ti Boost, but not a 660], then he continues on to link a GTX660 non-Ti, so I'm just going to assume that he's talking about the GTX660 non-Ti as that would be the card you'd compare to the HD7870 anyway).
Now, I know that if you look at most of the reviews out there you might see the GTX660 performing better the HD7870 in some situations. They would trade blows depending on the game, however, since then, drivers have come a LONG way, granted for both companies, but I'd say much more so for AMD than for NVidia. It's at the point now, where if you compare the two cards today, with the latest drivers, you'd see that the HD7870 actually beats the GTX660 in almost every single game. Most of the time it only beats the GTX660 by a very small amount, but in certain games, it actually beats it by a lot and surprisingly even beats a 7950 and GTX660-Ti at times.
On top of that, the HD7870 is less expensive than the GTX660. You can find an HD7870 for as low as $169.99 with rebate or $184.99 out the door vs the GTX660 which you can find for $174.99 with rebate or $194.99 out the door (Current Egg prices).
Also, since the HD7870 counts as a "Silver" card in terms of AMD's "Never Settle" package, you get 2 Free games of your choice from the list of offered games with it.
Hands down, the HD7870 is the better buy.