STANirvanaIND

Bothered me enough that I ended up splurging for the GTX 980 (coming from a 770 4GB which was great). On most games the .5 GB of VRAM wouldn't make a difference (assuming your at 1080p) but when you start modding games this can change drastically (my Skyrim install runs at about 3.7 sometimes more). Again may not be an issue for most people but I also didn't want to be upset 6 months down the road and unable to do anything about it.

Probably not since an internal card gains the benefit of the laptop's built in antennae, not to mention data transfer rates on a well configured N network will out perform USB 2.0 transfer speeds (even though its theoretical limit is higher, 480 Mbps vs 300).

I bought a 7260AC for my Y400, and it seems like the best option available. Antennae configuration is already compatible and performance is great assuming you have the correct settings and drivers (some of the driver revisions I've heard had problems). Even if you don't have an AC network I'd still say go with the AC card, it's an ~$10 difference and more than worth it for future proofing.

42mm M.2's only go up to 256 GB currently, anything larger would be 60, 80, or 110 mm

The Y400 uses a single channel LVDS cable meaning the maximum resolution is 1366x768 @ 60 Hz. You can try replacing the screen but it'll have grey lines and other odd behavior at power on (essentially it doesn't work). You can try finding a compatible dual-channel LVDS cable (maybe the one from the Y410, although I'm not sure) and then from there any compatible display panel should work (maybe the Y410's, I've heard some of the T series displays should be compatible as well). Make sure the mounting points are also the same as the original display. I'll say this though, I haven't heard of anyone having success doing it yet...