I wouldn't get too hung up about memory usage. The most important thing that will chew battery and slow down your device is CPU. And that is going to be up to the app developer and how they design things. Ideally, you want to write your program so that it doesn't run all the time - that is, it has a sleep state. If all your apps play nice and sleep when they are put in the background, your OS can then go into a hibernate mode when not being used by you (not tapping on the keyboard or doing other things interactively with it). This saves battery because hibernate allows halting the CPU cycles (when the CPU is awake, when it is not doing anything related to any app, it will be executing the equivalent of a NOP, which still chews battery). Also, if you have programs that don't allow a sleep (or rarely) , other apps get starved of cycles, hence you see a slow down.
As for your question as to why you have other processes at boot, some apps are written to start at boot. That was how the developer intended it to work. And some of these things, you don't want to kill. Some of these are even part of the core Android or HTC Sense, and if you kill it, Android will die an ugly death - try killing HTC Checkin and see how your Hero takes that!
If your system needs RAM, it will start swapping inactive app data out of active memory onto the swap. Use "free" in busybox, to gauge if anything got moved out to swap.