You have to remember the motivation, personality types, and socio-economic status of those who go with entry level systems. They are more commonly the type to buy something mid-range every 3 years and are less likely to upgrade or be concerned that they don't have amazing performance.
I could be wrong, but I don't see them as 'needing' 2 gig until around 2007 or 2008 because they just won't be pushing the resolution settings to really need it. As to load times, well 2 gig helps, but not enough to warrant the increase on a low cost build.
I'd actually be surprised if someone on an entry level system today purchased Vista much before late '07 due to its cost.