The differences are 120 vs. 320 GB HDD, 1 vs. 2 GB RAM, and $200.
The difference in shared VRAM is because of the installed memory and independent of which Mac mini you buy. If you put in 1 GB it's 128 MB. If you put in 2 GB or more it's 256 MB. It's automatically adjusted by the system.
Make sure those 2 GB from your MB are DDR3 SO-DIMM (IOW is it from a unibody MB?). DDR2 SO-DIMM (from plastic MBs) won't work in the new Mac mini.