Hey guys, I had a couple of questions about steam.
1. Depends on the game. Not all of them transfer. A lot of them do though.
Sorry, I can actually answer this question better than I did.
VAC is only relevant in games that use it, and any game that uses VAC will be Steam-only. most games use Punkbuster or their own anti-cheat. Regardless, the conditions for banning will be the same across Steam and Non-Steam versions of the game.
2. Already covered above in the FAQ posted by crazyleaves.
1 - Only certain games work. Here's a list of games that allow you to activate on Steam...
Don't know about multiplayer, but of course you can use whatever mods you want in single player games. VAC is an anti-cheat system that can be implemented in multiplayer games if the developer uses it, it's not some over-arching steam security system or something.
I like steam way better than retail games. Having a big list of all my games in one place and never needing to hunt for a disc is great, not to mention all the great games that don't have retail counterparts.
Some CS:S graphic mods are kinda gay, I got a one week VAC ban for something stupid and I never noticed it. One of my skin packs had a script to change how one of the gun works....and I didn't know until I got hit with it. They understood though and I wasn't the only one who got hit.
Just be careful with CS:S packs as some change the way weapons work be it for realism or random stuff.
All the right answers are in this thread already, but:
For retail games not download using steam, how would you go about updating them with dev released patches etc?
1) If it's on this list, you can register it in Steam.
2) VAC only affects multiplayer games, so your examples listed won't have VAC. At all. Secondly, even with VAC enabled games, you can mod them as long as you don't modify the core files (the .dlls and such like). To use an example from TF2, if the mod installs in steamapps/username/team fortress 2/tf, it won't attract VAC's attention since it's not modifying core files. Your mileage may very, however (Modern Warfare 2 has a strict no-mods policy, for example). I also suspect you are misinformed about the extent of VAC bans: they're only server bans.
3) Depends, although I tend more towards retail.
4) Retail patches depends on the publisher, really. For example, more recent EA games (i.e. Spore and later) will have an automatic updater included with the game. For something like S.T.A.L.K.E.R, however, you'll just have to resort to Googling and downloading the patches from GamersHell or GameFront, or FilePlanet or similar.