If you're building a fairly standard web app you probably shouldn't be worrying at this stage. Keeping memory usage low does not mean a significant speedup necessarily. It means your memory usage will be low. Sometimes you trade one thing for another depending on what you're doing.
When the app is written, if you run a load test (or you can let your users be your load test, like Twitter), there will be a bottleneck for some n of users. Remove this bottleneck, which could be cpu, database, memory, connections, bandwidth, etc., and there will be a new bottleneck at some > n users. Rinse, lather, repeat.
If you optimize now, you're most certainly optimizing the wrong thing. When I load tested my first production rails app, I found cpu was the first bottleneck I hit, not memory.
When the app is written, if you run a load test (or you can let your users be your load test, like Twitter), there will be a bottleneck for some n of users. Remove this bottleneck, which could be cpu, database, memory, connections, bandwidth, etc., and there will be a new bottleneck at some > n users. Rinse, lather, repeat.
If you optimize now, you're most certainly optimizing the wrong thing. When I load tested my first production rails app, I found cpu was the first bottleneck I hit, not memory.