iphone 3G iOS 4.1.2 Cydia essential upgrade & installing apps

Since I got my gf's old iphone 3G last week (locked to a Swiss carrier), I decided to get it unlocked so that I could use it in Indonesia (where I am currently working). The store used Cydia and ultrasn0w, as far as I can tell (and from what I have been reading on iForums for the past week).

I am very new to iPhone (the only i-Experience I have it i-Tunes on my HP laptop)- so please forgive my ignorance.

The phone is working fine, but I cannot get some (read: most) of my favourite would-be apps, such as facebook, skype, etc. because they require iOS 4.3 or higher. And I only have 4.1.2 ... So from what i read around, I could find these kind of apps on Cydia... Is this correct? How can I do this?

My main question though, is when i open Cydia, it is asking me to perform "3 essential upgrades". I've looked on many forums but nowhere they discuss what happens when Cydia is actually asking me to remove ultrasn0w (isn't this the program that ensures I can use this phone here?)... It wants to do the following:

I selected "only essential upgrades", which include:

Install:
- bigboss icon set
- cydia translations
- debian utilities

Upgrade:
- base structure
- core utilities (/bin)
- cydia installer
- UIkit Tools

Remove:
- Big Boss's Source
- Cydia Community Sources
- Dev-Team
- ModMyi.com
- Ultrasn0w
- ZodTTD's Source

Can I accept this without the risk of having to go through an entire process again?

The latest firmware an iPhone 3G can have is 4.2.1... and no higher....you won't be able to use most of the newer apps available in the Appstore or Cydia...