Easy way to tell if a low power MCU supports external memory?
The chips you referred to are in the TI 430 family which is a 16-bit architecture.  You are going to need either 32 or 64-bit chips to support GB of external memory.

If you want the external memory to be mapped into the address space of the MCU, then you need to find one that has an MMU (memory management unit).  The widest selection of processors that have MMUs (besides the 80x86 and AMD processors used in PCs) are the various ARM architectures.

For RAM, You will want to look at various ARM processors with external memory interfaces, such as 32-bit DDR3, which is the same kind of RAM you might find in a PC.  You should search for "32-bit ARM DDR3" or similar.

Here is an example of an ARM processor with such an interface, and here is a document that explains the memory interface in detail. This is an older processor (ARM5) and older memory (DDR2), but I chose it because of the good explanation of the processor-memory interface.

Most Linux systems boot up from some a bootloader, and then copy the Linux kernel etc. off of a filesystem contained on an SD card (for example) into RAM to execute.  If you added a battery back-up system to your external RAM, you could make it non-volatile, and not have to boot up every time.

Its not clear from your post whether you want external RAM, Flash, or both.  In addition to the RAM options you can also check out external flash options by searching for "ARM ECC NAND flash".  (The ECC stands fro error-correcting code, which is needed for large flash memories.)

Unless you really really want to build your own Linux-compatible system, which is going to be a lot of work, I  suggest you look at one of the existing open-source systems such as BeagleBone Black.  It has less memory than you are looking for, but since the design is open-source, you might be able to expand it.

Typical Linux systems have maybe half a GB of RAM and a couple GB's of Flash.  What is your application that requires 4 to 8 GB of external memory?