Can you patch the games you buy here and more importantly, do you need to?

Most games on GOG should be "patched" versions to run out of the box. However, Carmageddon has an option patch in the subforum that GOG provided in case people want to play the Windows version. The Carmageddon installer only sets up the DOS version.

Modding is a whole different issue. There are threads like this one below in which GOG advises different mods and patches for a game like Baldur's Gate.

http://www.gog.com/forum/general/enhance_the_gameplay_in_your_edition_of_baldurs_gate_from_gogcom

Thanks for the quick reply. I'm not sure how to mark this as answered but it is so, ummm, how do I do that?

To expand on JMich's answer. For newer games, the companies have to provide patches, which you in turn download from here.

Things seem to have gotten a lot better with devs providing proper standalone patches than when the site started out.

Yes, you can. Unofficial or community patches work great for games like Gothic 3 or Thief's saga, also.
Generally, games are updated with the latest official patches.
If you aren't sure about it, check the specific game forums where you can find (or ask about) the version that comes with the GOG release of the game.
Hope it helps.

Most of the time the game is patched up and ready to go.

Sometimes GOG includes a patch that should be applied.

It is helpful to check the forum of a game to check if there are any sticky threads that point out possible fan patches that could make the experience better.
Also, some games have helpful mods that can be applied to improve the fun.

As a rule of thumb, check the forum for a game before you play it.

With older games, there's usually a "Patched to version ♥♥♥" info thing on the game cards, at the very end of the Minimum system requirements area.

No idea, since I haven't asked any questions here yet :P
There should be a "Mark as Solution" button near my post though, unless that option is near the first post. Someone else should be able to answer this question though, just wait a bit.

I have a whole meta-game I play with my GOG games.

After I download them I go to the game specific forum and read up (there are usually stickies) on how to set them up / patch them / mod them to get the best out of them.

This is particulalrly useful for the older games where there are often hi-res patches, missing content mods and occasionally even whole new engine replacements available.

I then put them on my backlog to be played "real soon now".