i think you cannot go higher than 960M within the price bracket. Easily last 3 yrs? hard to say considering it's not very high on Maxwell based mobile GPU hierarchy and there's already Pascal stuff out, full desktop version no less. If you haven't already, check out the detailed review here:
http/www.notebookcheck.net/NVIDIA-GeForce-GTX-960M.138006.0.html
Seems it's able to run modern titles on med at 1080p or in some cases a lower res. and achieve playable framerates. So, you can take that into consideration when trying to figure out how it should fare a few yrs from now. i'd estimate you could be able to run future titles by tinkering with the graphics settings and making compromises on visual quality/resolution.
that said, following options from MSI, ASUS, and ACER are available:
https/www.amazon.com/MSI-GL72-6QF-405-NOTEBOOK-i7-6700HQ/dp/B01AC40ODQ/?tag=tdrvr-20
https
/www.amazon.com/GL752VW-DH71-17-Inch-Discrete-GeForce-Metallic/dp/B01578ZK9A/?tag=lapninj-20&th=1
https/www.amazon.com/gp/product/B015XBKYB0/?tag=lapninj-20&th=1
they are more or less the same spec wise, though ACER has a 960M w/4 gigs. on the other hand, it features a slower HDD, which makes it almost identical to the DELL you shortlisted
https/www.amazon.com/Dell-Inspiron-i7559-2512BLK-Generation-GeForce/dp/B015PYZ0J6/ref=sr_1_1?s=pc&ie=UTF8&qid=1474289164&sr=1-1&keywords=Dell%2BInspiron%2B15%2B7559&th=1
When it comes to gaming laptops, i would consider all of the above mentioned brands before Dell but that's just me. However, Dell is very close spec wise so you can definitely choose that if that's the brand you prefer.
regards