8600gts- Duel or single monitor?

I have a 8600gts (works great with flight sim x by the way) and i am looking to buy a monitor(s) and i want to know if having duel monitors will slow down my gaming???

It won't affect games at all. Your secondary monitor will simply lock up whenever you launch a full-screen 3D app.

Certain games support multi monitor setups. (isn't flight sim x one of them?) If you attempted to play these games in this mode, it would of course affect performance. You can expect half or less performance, as both the GPU and the frame buffer (and VRAM) have an increased workload.

I'd just keep it single IMO. Lots of times the dual monitor setup is just a gimmick, like the secondary monitor is a map of the battlefield. (Sup Com)

Edit: if you don't mean multiple monitor gaming, then look below. I didn't use my head when posting, lol.

it won't affect gaming. I run a dual monitor set up, and I usually just use the 2nd monitor for various things like monitoring temps while ingame or ventrilo/teamspeak.

It won't lock up, but it doesn't draw much resources as it's not running anything 3-D. I use my second monitor to see who's talking to me on xfire, aim and keep an eye on the time.