6GB of memcache seems a little low for a decent sized installation, especially if using memcache as the caching driver. Even if you have 32-bit machines, you can still get 3GB+ per machine easily. Or, better yet, take the splurge and buy a couple of cheap 64-bit boxes instead. The great thing about memcache is that it doesn't need powerful, server-grade hardware to run. In fact, that is overkill. Any cheap 64-bit box is going to be plenty powerful enough to handle the memcache load. Quite honestly, your bottleneck will come from the network traffic to/from the memcache server/PHP server, and most likely the network infrastructure will be the limiting factor, not the network I/O speed on any given interface card.
Also, absolutely no reason to reboot memcache every night. In fact, I highly recommend you don't, especially if you are caching with memcache also. Rebooting simply loses all those hardearned mailbox/message caches that your servers labored to build every day.