I decided to use FeedTheBeast as an alternative, since it has IC² and many other interesting mods and allows non-involved people to easlily mod their client.
But I'll try ForgeBukkit anyway !
Thanks for all your answers once again, applied [Solved] tag. See ya ! 
Last question and I'll be closing this thread : Can a server be ran with both forge AND bukkit ? Reason is, I'd love to add some more mods but can't find any proper "non-bukkit" ones.
Ok, so after a BSOD and some faily testing, it finally works in dedicated server too.
My most sincere thanks to all of you, I'll be able to make some clear tutorials for non-english speaking french players.
Thank you again !
Wow !
I wasn't expacting so many (and so kind!) answers 
Greatest thanks to you all !
I do not particularly use 1.2.5, but I read somewhere that it was the latest IC² server build available. Internet is full of people talking crap, so I thought I'd better ask qualified people about it (E.G. you all 
)
I'll try this out right now, and let you know.
[Edit]
Ok, so, I diligently followed the steps detailed by Epic Lulz, and here's what I got.
Quote from Epic LulzDisplay MoreFirst thing to note is that IC² is now on MC 1.4.6, and unless you will zip
up a copy for MC 1.2.5 (you will end up with a black screen). Secondly,
Forge has separated from ModLoader and ModLoaderMP, however mods using
those can still be ran on Forge, but Forge mods cannot be ran on ML and
MLMP. Using them together will cause crashes.
How to set up client:
1. Force Update to clear your minecraft.jar Ok, right.
2. Delete META-INF folder. Done
3. Download the latest Forge and drag the contents into the minecraft.jar. Do not delete the META-INF that comes with Forge. Ok, cool
4. Install Optifine by dragging its contents into the minecraft.jar (optional, but recommended) Trusing you, I did that too.
5. Launch Minecraft And it works
6. Close Minecraft And it... well, closes, basically
7. Install IC² by dragging its jar file to the mods folder. Done
8. ???
9.ProfitRun Minecraft, wait before FML checks some few things, get the Mojang screen, and then... black screen.
Reading carefully what you said once again, I understand that I have to use MC 1.4.6. Trying it out.
Ok so after I got that I needed all in 1.4.6 (Forge, IC² AND Minecraft), it finally worked in SSP.
I might have done something wrong, because every time I'm crafting an IC² item, it POWERLAGS like hell.
How to set up LAN server Actually it's the dedicated server how-to that I need, but thanks for that part anyway
1. Finish setting up client
2. Launch Minecraft
3. In the world you want your LAN server to be in, open options and click Publish to LAN and select the necessary options.
4. ???
5. Profit
How to set up dedicated server
1. Place the minecraft_server.jar in your Server Folder So, err, empty folder, right ? Right.
2. Place Forge in the Server Folder. (DO NOT DRAG IT INTO YOUR SERVER JAR) Ok, did that.
3. Run Forge. This will be how you run your server. Here, I quite didnt understand. The Forge file I got from their website, and used both for client and server as advised by Wliu is a .zip archive (minecraftforge-universal-1.4.7-6.6.0.499.zip, precisely). And, apart from extracting files from it, I quite didn't get how to "run" it
[Later]
I guess that I'm gonna need the 1.4.6 minecraft_server.jar too... Any idea about where I can find it ?
4. Close your server
4. Drag IC²'s jar file to the mods folder.
5. ???
6. Profit
Hi there !
I've been using the old IC years ago, really loved it, but since modloader decided that it hated me once and for all, I kept the original minecraft until now.
I'd love to set up a server running IC², but I need some light on specific spots.
It's about versions.
The main issue is : if I want to create an IC² server now, what Minecraft version (both client and server) should I get ready for modding ?
Second problem is forge. I dont know (or couldnt see it among the huge amout of information available) which version I should get.
What I seem to understand is that I need the minecraft_server.jar v1.2.5, same for the client, and the last available Forge build for these two (which, according to their site, is 3.3.7.135).
Is that right ? Or maybe the newest Forge build will work anyway ?
What I used to do for now is :
1°) Getting minecraft_server.jar
2°) Getting Forge
3°) Opening both archives with WinRar
4°) Dropping all the Forge archive's contents in the server.jar
And actually stopped here because the server wouldn't start even with only Forge installed. But I guess if it had worked, Forge would have created a "mod" directory (in the server main folder) in which I should put the right IC² jar server file.
I'm not asking any help about installing Forge (though I guess I'm gonna need some anyway ), but I thought I'd figure which versions of Forge AND minecraft are necessary if I want the most recent IC² server properly running.
I hope all this is clear enough... If you need any more details/rephrasing (since my english is far from perfect), do ask 
Oh, and I'm running on Windows XP sp3, if that matters.
