To answer your second question (how much cache you need) we have found no benefit to having more VRAM than the minimal number it starts with. The problem is testing it is extremely difficult, you don't get to buy a card at 1.4GB VRAM for example to see if it actually has problems, but we aren't getting reports of 1.5GB cards choking so while its absence of data it neitherless suggests that the caching is largely unnecessary.
No one has done a detailed study however, it may be you see more variance with having a lot of cache available, but without any real data or reports suggesting there is an issue its not a big deal if there is an effect. Given the numbers you have provided I would expect a 3GB VRAM card will do brilliantly at the resolution you tested, but its probably too much for a 2GB card.