Ok, number one: don't update! If you install 4.0.2 you'll lose your jailbreak.
Everyone has their favorite apps, but some great basics would include SBSettings (gives you a pull-down menu to control all sorts of things you'd otherwise have to go into settings to find--wifi, brightness, task killer, 3g on/off, etc), BiteSMS (lets you text from your lockscreen, among other nice things), and Activator (lets you assign different gestures to different apps--for instance, right now I've got "Double Tap Status Bar at Lockscreen" set to open my BiteSMS quick compose window).
Winterboard is what you'll want to theme your UI, but some of it is still buggy and/or themes haven't been updated yet. My3G is for Facetime over 3G. There are a ton of ringtones and other little tweaks that make it fun; see Make It Mine if you'd like to change your carrier name to something more personal. Most of all it's just fun to poke around Cydia and see what's available. Enjoy!