Nothing holds its value, everything is obsolete the second it hits retail. And just because a particular socket stays around for a while doesn't mean it works with everything for that socket - for instance socket 775 mobos for the Pentium 4 cannot take Core 2 CPUs.
What do you want to do with your system? Unless you're doing heavy video/media work you won't get the most out of a socket 1366 system. I think there's a lot of fluff and debate going on regarding if i7 should've ever come to socket 1156 in the first place given there's not much difference between i5 and i7-1156, so I personally couldn't choose between them - there are factors like Turbo mode and Hyperthreading, but that's real fine tuning stuff when you consider what your system will do.
There's also a hooha going on regarding the actual CPU socket on most 1156 motherboards being shoddy and not giving proper grip to the CPU, causing poor contact, higher power draws and some pretty fireworks when overclocking.
Using 9800GT in SLI isn't going to saturate the PCI-Express lanes in a socket 1156 board so you've nothing to worry about there, but the P55 chipset isn't super fantastic if you're running top-end cards in SLI or Crossfire (somebody else can dig out the benches for me hopefully - sorry!).
Finally, with the 6-core i9 coming to socket 1366 it'll be interesting to see how far the mainstream 1156 will go in the future.
So, I guess the conclusion is this:
For a gaming system with 1 top-end graphics card, go i5 on socket 1156 (and whack in 8GB RAM if you can because 4GB DDR3 dual channel kits aren't too expensive) and go Radeon 5870 when you upgrade your graphics later on.
For a gaming system with top-end multi-GPU set-up, or heavy media creation work, go i7 on socket 1366 (6GB triple channel for gaming, 12GB triple channel for media or if you can afford it anyway).
You can save a few bucks over the i5 if you go Phenom II, but others will have to advise you on that as I'm traditionally an Intel boy (media creator first, gamer second).
Incidentally, if you do go i7-1366 there's no need to go any higher than the i7 920 D0 stepping - with a decent cooler it'll clock up to 4GHz easy and thus you get the same performance as the more expensive chips, even the Extreme 975.