How can a UXD / UI designer assess the back-end effort involved when designing front-end functionality?

I'm a Web designer who sees the incredible potential to deliver solution-oriented and focused web applications incorporating Web 2.0 technologies. I understand conceptually how these Web 2.0 technologies work and what possibilities they create. Usually I work in a team environment, but I have been venturing on my own to design the front-end functionality for a couple of web 2.0 applications that offer networked communities, personalization and highly interactive solutions in niche areas. One application in particular has a high penetration potential (i.e., millions of users collabrating on projects) and would solve a unique industry problem. I intuitively know that this effort could be complex from a database perspective as well as from an infrastructure perspective, yet I also know it's possible because of the technologies and applications that are already available today. Do you have any advice on helping me scope out what resources the back-end development effort might require?

Well, MySQL can handle lots of information and still be fast. It works well with PHP, and PHP is easy to learn and code. You probobly want a good control panel for coding and management. I recomend Plesk, just because it's popular and seems to fit most peoples needs. Other than that, just make sure you've got a nice big server that can take the crowds (lots of bandwidth too). :D