Usually, a good portion of new android developers have trouble with Eclipse.ini settings as java VM settings have changed over the years and there is a mess of legacy and new settings. This gist of my eclipse.ini settings is for multi-core and tested on MS Vista 32-bit, AMD 2-cpu-core 2.6 GHZ, 4 gig ram, and Eclipse 3.7.
It seems that now the choice of GC is more important whereas the G1GC is somewhat problematic as far as performance. I ended up using -XX:UseParallelGC instead. To give you an idea, about 20 small projects in workspace and startup time to using the IDE is under 1 minute. Obviously, the more cpus would improve performance and bigger projects will impact the startup time due to the ADT plugin scanning/building the projects on startup.
On Linux with JDK6 I had used G1GC settings in the eclipse.ini and that was problematic as far as performance on the same exact computer hardware so I assume that changing to using ParallelGC instead for Linux should also work.