You can't modify all games the developers have to provide the SDK for you to be able to modify them. For example all valve games, i believe use the source engine. So to make mods for games that use the source engine you would have to download the source SDK from steam or some where else. And to get you started here's the wiki to the SDK

http://developer.valvesoftware.com/wiki/SDK_Docs

The Unreal Development Kit(UDK) was also realeased recently so you might want to take a look at that as well.

Well some people do play a lot, but sometimes stats are misleading. For example if you just let the game on in the menu and dont really play it, it will show on the stats, that might be a reason why you see some inflated play time stats.

The community has always been active, and will be active for many years to come like counter strike, and cs source have been. They still need to update some classes though.

I wonder why there aren't too many co-op rpgs? would it work in a game like oblivion, mass effect, dragon age? I think it would be more fun to play these games with other people, agree?

once downloaded you just click on the game from steam to play it. No installer and yeah you can back up your games to a dvd, but i never done that. Once you buy a game it's tied to your account forever. So you can re-downloaded or use backup dvd which would be easier i suppose.

That cpu is fine, just needs a little over clocking that's all. It can reach 3.0ghz up to 3.2hz with the right cooling.

And if you really want to improve that rig spend the money in one of the new gpu's that came out. But the hd 4850 could still hold its own