Trisquel 64 bit won't have any problems with 16GB of ram.
Don't go with an NVidia chipset as it won't have any free software support.
An ATI chipset is your best bet. ATI released the specifications and code
needed for good support of its graphics chipsets. The current version or next
version of Trisquel should see acceptable levels of support for at least some
graphics cards.
That is not the only thing to worry about though. You also need an Atheros
wifi chipset.
Those are at least the two main things you need to be concerned with.
Also, be aware that Dell, Tosbia, Lenovo, and HP ship laptops with digital
restrictions on the Mini PCIe slot so you may not be able to replace an
incompatible wifi card with one that works. I am not aware of any others that
may be a problem at the moment. I believe most other laptops shouldn't
present a problem in this area though.
