If you are experiencing spikey lag, java crashes or out of memory errors, try adding this to your JVM command line options in the launcher profile:
-XX:MaxPermSize=512m
If you are using NEI Plugins, be sure to use 1.1.0.6, as the previous versions will crash you.
Also, if you are using an AMD / ATI video card, make sure you are on Catalyst 13.9, and nothing newer. There are a number of crashes reported in Minecraft on the the newer drivers.