Honestly, the 870M will never surpass a 880M in terms of raw potential. The 880M is a better GPU in terms of strength (though nVidia apparently has a few issues they need to fix with it, but they're working on it right now as far as I know) and the 870M is basically a side-grade to the 680M. The 680M had very slow memory and core clocks. The core was able to go up pretty high, but the memory was not heavily overclockable due to a design limitation.
The 870M is what happens when you take a 680M and bump the default clock speed from 720MHz to 941MHz and the memory from 3600MHz to 5000MHz. The downside is that the memory bus went from 256-bit to 192-bit. In effect, it's the same difference performance-wise; the 680M usually limited itself to 4400/4600 maximum memory clocks while the 870M will easily hit 6000 on the memory clocks. Now the 880M has 5000MHz on the memory and can hit 6000MHz on the memory easily too, but the memory bus is 256-bit there, so the memory will always be better. The core speed too, will likely be a lot better because even at the same core speed, the number of cores is less on both the 680M and 870M versus the 880M (1344 versus 1536).
To summarize, more cores = more items to process graphics. So if you had 9 clones of a lumberjack building a house, 10 clones of the same lumberjack would build it a bit faster.
As for memory, think of it like cars on a highway. If you have a wider highway, more cars can pass. 680M has a wide highway but each car is travelling slower than the 870M. The 880M both has the faster cars of the 870M but also has the wider highway of the 680M. I hope this lets you understand things a bit easier.
As for performance... the 880M versus the 870M, both cards at stock and at stock vBIOSes will likely run similarly. When you get proper vBIOSes under control (which people are working on now) and the throttling on the 880M stops (throttling is slowing down of core speed to handle heat, something which does not work correctly right now on stock vBIOS) and the drivers etc work great, the 880M will be by and large the better card to have.
If you want, the 870M with a 6000MHz memory clock (and maybe a slight OC on the core) will perform almost exactly like the GTX 660Ti desktop card. If you think the 660Ti's performance is good for you and won't leave you wanting, then go clear for the 870M and save yourself some money and we'll help you with OCing when you're ready.
If you want the 880M's benchmarks; with 6000MHz on the core and stable clock speeds, it's pretty much 13MHz slower than a GTX 680. So you can use a 680's benchmarks to see how strong the 880M is. If the 680 benchmarks are something you're more interested in having, especially if you're willing to possibly do a couple repastes with really good thermal paste etc to make sure your temps are fine, you can even go beyond that and consider your card to be inbetween a 680 and a 770 in terms of raw power... but with 8GB vRAM.
In the end, it's all on you to decide, but I hope that clarifies the power that each card is capable of bringing to you, and I hope you can use those cards' benchmarks to decide for yourself which you would want more, or if the 880M is justified as being that much more expensive than the 870M for you. Personally, I went with my two 780Ms because I wanted my laptop to last me a solid 4-5 years, and I'm dead certain I will still be playing games on "high" 3 years from now at 1080p. So also think about how long you want to keep this machine as your primary machine, and that may influence your decision a bit more =D.
Problems? See this thread at archive.org.
