There's a quick graphics option in the settings - makes leaves non-transparent and a few other things that might result in better performance.
(Also, as Chris says, set the Fog / view distance to one less than max and that'll help without ruining it too much.)
During the night, work on expanding your base - what you do depends on what style of structure you have, and how much you care about decorativeness.
* Back door / escape exits - if there's a creeper sitting outside, you might want an alternative way out - or (if you've been delayed far away) a quick but safe way in.
* Furnaces. When you get to smelting anything, you want to have plenty of furnaces on the go in parallel, otherwise you'll be waiting around for ages.
* Spare tools - build a large storage chest and fill it with picks, axes, swords, and whatever else you might use - saves precious daytime if you make sure you have a good stock and can just grab them into inventory.
* Spare items - similarly, fences, ladders, steps, boats, etc - harvest a huge bunch of wood then spend a night building a bunch of these in advance, and plonk them in a storage chest for when they're actually needed.
And of course, once you've managed to get at least 26 iron, you can build yourself a full set of armour and a sword and consider not cowering inside at night.
:)EDITED: 21 Jan 2011 00:50 by BOUGHTONP