Right so I'm an avid Steam user and launch all my games through Steam so I can use the overlay. Only recently have I realised that the overlay dosn't work with Fallout. I'm assuming that this has something to do with the resolution (i'm not using the resolution patch). Does anyone know how to get the overlay working without using the resolution patch?
Cheers!

Using the Resolution Patch.

Ah right. Thanks!

What is this Steam overlay?
I'm thinking about buying Fallout 3 through Steam, but I've never used it before, I'm suspiscious about it. Do you know if we have to be online or launch steam to play the games?

If you buy Fallout 3 on steam you will have to download it and launch it once and after that, you can play Fallout 3 anytime just by launching steam and launching fallout 3 from there (online or off).

Well, once you purchase the game it will download through steam and save it in "Steam/steamapps/common". Will have the same files as the retail version... Including its own "Fallout3.exe" (or something). However, the executable calls Steam to check if your account actually owns the game.
You may think this is very restricting, and requires to be connected to the internet all the time, but there's a nice feature which allows you to use Steam "offline" as long as you want (I was cut from the rest of the world [no internet connection] for many weeks and that mode worked without issues). Of course there's always the risk you suddenly lose your connection and can't access your account anymore, though Steam lets you install the game on as many computers you want (one account at a time running!).
It has it pros and cons as you can see, I'm using it for many years and am not disappointed.
But don't take my word for it, read through some documentation and the FAQ of steam.
Steam support link: https://support.steampowered.com/
Steam Forums: http://forums.steampowered.com/forums/
Fallout 3 Steam forums: http://forums.steampowered.com/forums/forumdisplay.php?f=472
Steam Wiki entry: [url=http://en.wikipedia.org/wiki/Steam_(content_delivery)]http://en.wikipedia.org/wiki/Steam_(content_delivery)[/url]
Hope this was helpful!
EDIT: Fixed the links

Thank you very much, guys.
I think I'll give it a try. I prefer gamersgate and GOG.com ways of doing things, but I guess I'll have to settle with Steam, because I don't think Fallout 3 will be released here in Brazil.

I'd reccomend buying the boxed game, just because Steam forces you to update games, and right now a lot of people are having problems with crashes. Plus if/when they release the tool kit, it might be easier to run mods. You can open shortcuts through steam though if you HAVE to use the overlay.
As far as the overlay, you need minimum DirectX 7 to use it, and the fallout games use I think 5, so no luck :(.

Well, seems like they have forgotten about South America if what hito said is true. (no mention of a release date in S america on tthe wiki page too...)
Maybe considering an import (if that's possible) is an option, but I bet it would be more expensive than purchasing it through Steam...

Much more expensive, import taxes here are of 100% for games.
Unfortunately, Brazil has a pirating tradition that scares game distributors away. I think only EA launches their games in here.
Now I'm concerned about the modability issue under steam. I thought their games had the same files as the boxed ones. How is cheating handled under steam? I heard about cheating ban, but I imagine it's only for multiplayer games, right?

yes, that is correct.

Cheating is handled by VAC (Valve Anti-Cheat) Servers. If you cheat on them, expect a ban.
If you do desire to cheat, you can use a non VAC server. You still might get kicked though.
There are a couple of games that use other anti-cheat methods like Punkbuster. Painkiller comes to mind. These will not affect your actual steam account, but use whatever method punkbuster is set to.

Being that Fallout 3 isn't an multiplayer game, whatever you do to it shouldn't matter. While there are some issues with some games and mods, I believe if you were to download FO3 through steam, and then launch it with fallout3.exe, as opposed to fallout3launcher, it bypasses the steam check.