In the LDS Church, who decides where to place new Churches, and how? Fairly straightforward question:  In the LDS Church, who decides where to place new Churches, and how?  Are there specific criteria? Is there a process for determining whether a Church can be supported?

I'd imagine that the process has something to do with how many believers there are in a community.  I know that witnesses go out everywhere, and in my experience, I attended LDS services in a high school gym, and there was talk about a "real Church" being established, but I never really thought about it in detail, so I didn't ask.  I'd guess that there's a criterion for how many believers are in an area to make it worthwhile to place the building there.  (After all, people can be taught just fine in a high-school Gym.  A building seems to be a convenience rather than a need.)

What is the actual process, and if there are criteria, what are they?
From page 6 of the Facilities Management Guidelines:

Factors that affect planning future meetinghouse needs include (1) the characteristics of typical Church units in the area, (2) past growth rate and patterns, (3) projected growth, (4) planned unit divisions, and (5) the size, location, and extent of existing building use

Matt is also right; on page 1, it says:

Planning New Facilities
Area administration office staff maintain plans for meetinghouse needs in all stakes. The stake president is asked to update the information for his stake in those plans regularly. For matters pertaining to new meetinghouse space, the stake presidency consults with the administration office.

Meetinghouses used to be funded by individual units, but are now paid for by tithing. Sacrament meeting attendance has been used to measure the need for a building in some wards I've lived in.
For instance, the Chinese branch in Manhattan met in a one room office until they had enough people attend that they became a ward and got their own chapel. However, because it was just one small ward, the church was one floor of a building. Meanwhile, Harlem had several wards, and their growth caused the LDS church to build a new meetinghouse there (all this was from 2003-2005 or so).
Likewise, temples are built according to the number of nearby active members (paying tithing, attending church) and demand on other nearby temples.