Well, I just put it here too, for other lazy bastards like me:
# db_shared_buffers: 128MB for 1GB, 256MB for 2GB, or 256MB * GB, max 4096MB
# UNICORN_WORKERS: 2 * GB for 2GB or less, or 2 * CPU, max 8
BTW, on a 4GB dual-core box (Intel Xeons) I’ve settled with 5 unicorns, as suggested by your awesome Matt Palmer. This shoul take you somewhere around or below ~100K daily pageviews before choking (heavily depends on your users’ behavioral patterns). It delivers 50K pageviews of traffic with nominal load and good responsiveness.