Memcached? Only vertically? Horizontal scaling is built-in in all memcached clients.

yes, I was just saying that we can't wait for replication, when someone places a bid all the others users must have that information immediately.

memcached doesn't do replication as far as I know. It will just have to do some requests second time. It still beats re-calculation on each request.

Replication is the wrong word for it. Key distribution is what you want. If a given memcache server goes down then the keys it was storing should get reloaded to a different one in your pool by your cold cache loading code.