even if it is, in a few months time, or maybe next year there will be games that a 7870 may find hard getting 60fps. games that you might find fun to play. so if you can afford it, get it. it's not like you'll only be playing facebook games.
getting constant >60fps is nice, or even 120 if you have 120hz monitor
Look at it this way AMD Radeon HD 7950 offer performance that is nearly as good as that of the high-end models, but at a more modest price. If you stick with a 1080p or 1920 by 1200 display with one of these cards, you should be in good shape.
$200 to $300
At this level you may have to start making some sacrifices in detail settings. For GPUs such as the AMD Radeon HD 7870, that means running at "high" rather than at "very high" detail level, and it almost certainly means disabling antialiasing.If you do not mind that then get the 7870 but generally i recommend you buy the best you can afford that way it will last you a good while and you won't need to upgrade so soon.
So answering to OP question - it depends on the resolution You play at. And that's the first thing You should give us before we give You right answer
No I do not think it would be overkill. Even if you are running at a lower resolution than 1080p you would have a card that would be ready for higher resolution when you upgrade monitor's. I have a rig with a Sapphire Hd 7950 and can tell you it is a great card.