Place files into the Steam downloads folder instead of re-downloading them?

I recently bought Dark Souls on Steam. I already have the two hefty 1.5Gb files that make for the bulk of the game and I really don't want to download them at 100kb/s.

I found the folder where Steam was saving the game to, replaced the files, but Steam didn't update the download progress bar when I started it back up - understandable.

Now, is there a way to have Steam re-check the file 'completion' so to speak so I don't have to download it?

If you already had access to a downloaded Steam version of the game, you could do this, but it doesn't always work with other versions of games. To be certain, stop the download, right click on the game and open properties, and delete all local content (after making sure you have your two 1.5GB files somewhere else). Then put the files back in the Dark Souls folder (you may have to recreate it, so write the exact folder name down somewhere). Now tell Steam to download it again. There is a step in the download where it checks for existing data. If your files will work with Steam, it will detect them now. Otherwise, it sounds like you're out of luck.

It depends on whether or not the files are in the format Steam accepts for this purpose: Steam backups.

Here's for example my Team Fortress 2 steam backup:

F:\snip\Steam Backups\Team Fortress 2>tree /F
F:.
├───Disk_1
│       autorun.inf
│       steam.ico
│       steambackup.exe
│       Team Fortress 2_disk1.sim
│       Team Fortress 2_disk1.sis
│       Team Fortress 2_disk1_0.sid
│
└───Disk_2
        Team Fortress 2_disk2.sim
        Team Fortress 2_disk2.sis
        Team Fortress 2_disk2_0.sid

(Multiple disks are optional.) If what you have is like that then you can restore the backups merely by using the steambackup.exe file, or alternatively: Steam → Backup and Restore Games → Restore existing backup.

Steam backups are not bound to a specific user, so if you have a friend who has the game installed, he can make a backup for you and that should work.

If that's not what you have, there might still be hope under the form of a Steam Library, but the feature is new (still in the latest Steam Beta at the time of this writing) and thus unlikely to be of much use to you.

This is what a Steam library with Bit Trip Beat in it looks like:

F:\snip\Steam Library>tree /F
F:.
│   steam.dll
│   
└───SteamApps
    │   appmanifest_63700.acf
    │   
    ├───common
    │   └───BIT.TRIP BEAT
    │       │   (snip)
    │               
    └───downloading

If this is what you have, you can import it with Steam → Settings → Download and Cloud → Steam Library Folders → Add Library Folder.