I have web application that will be similar to forums. I plan to have few thousands+ users on this application, and i wonder if i can use MS SQL Express - here is the list of it's limitations:
Despite the cpu usage, i wonder if this 1 GB of memory usage is enough. Can anyone tell me if this might be enough, or maybe give me some examples when this 1 GB would be enough.
No, no one can look into their crystal ball and tell you if it's enough. What you need to do is put some load on your application in development and see how RAM usage scales with 1, 10, 100, etc users. If it's linear, you should be able to extrapolate 1000+ users pretty easily.
Every application behaves differently, since every application uses different data sets and queries. You need to profile your application and see if the RAM usage will be a bottleneck.