If you want to put them together(side by side) I would recommend getting the same exact monitor as you have now for a 2nd display, for a couple reasons,
1 They will line up with each other (widescreens aren't as tall)
2 The colors will match up perfectly.
3 When you use the same resolution on both, everything will line up from one screen to the other.
You can have both monitors on while gaming but, The game will only be on your primary monitor. The other monitor will show your wallpaper and anything you had on it before you started the game, like, Windows Media player, ATI tool or your system info.
If you want your wallpaper to go across both monitors like it's 1 monitor, You will need some dual monitor wallpaper, You know, rectangle wallpaper twice as wide as your resolution, for example 2560x1024 for dual 1280x1024 monitors. Then just set your wallpaper to "tile" in the display settings.