Firstly, there is an issue with the ADT plugin and Helios which causes lag with looking up Android classes - use Galileo instead (v3.5).
Secondly, the emulators become more resource hungry depending on the version of Android you're developing for. Example, I have a P4 2.4GHz, 1GB RAM PC with Win XP 32-bit and an Android v2.2 emulator takes at least 4-5 minutes to load up. An Android v1.6 emulator on the other hand loads up in less than 1 minute. Remember though that once the emulator is up and running, you can leave it loaded and it will be more responsive than first use.
Also bear in mind that if you give your emulator a 2GB SD card (for example) it will try to create that through virtual memory if there isn't enough physical memory.
