Why do you need to update?
If you have 10.6.4 installed and you want to update, then you would update to 10.6.5 or later up to 10.6.8. If you are going to update I would suggest you download and install the Mac OS X 10.6.8 Update Combo v1.1 update.
If you run Software Update (on the Apple menu) it will show you the latest updates for all your Apple software. And since the latest update for Snow Leopard is 10.6.8, it will most likely offer to install the 10.6.8 "combo" updater (rather than a series of the in-between ones like 10.6.5 through 10.6.7).
It also should automatically check for other updates after installing any that are prerequisites for other updates (like the Java security fixes that require 10.6.8). If not, keep running it until it says your software is up to date or it only lists things you do not want.
This is by far the easiest way to keep your Apple software up to date. It is a good idea to do that because it increases the security of your data & often patches bugs or adds new minor features to your Apple software.