http://www.minechemmod.com/
even though some mods are no longer in development, there are still good replacement mods and IC2 is still compatible with minecraft 1.5 and I am quite sure with 1.6 as well. Also I played on a server with Flan's gun mod and it works perfectly fine, creates no lag and works very well with multiplayer. Yes, Flan's Vehicle mod may have a bit of lag but it isn't that bad, you might have to sacrifice some computer memory but it's worth it. You can also easily get optifine for the slower computer so in this case, there is no downside.