SLI question

I recently added a 2nd GTX260 to my rig and set up SLI, and so far I have not had any issues. However, one thing I found strange is that the 2nd newer card has been labeled "GPU0" and the original card is now "GPU1". The original card is still in the same PCIe slot: the slot closest to the CPU. Does it matter which card is considered GPU0 and which is GPU1? Also, does it matter which card drives the monitor? I currently have my LCD plugged in to GPU1; would moving it to GPU0 make any difference in anything?

General info if needed: original card is a Zotac AMP! NVidia GTX260 (factory overclocked); the second card is a Sparkle GTX260 (I overclocked it to match the Zotac card); Asus M4N72-E motherboard; newest NVidia GeForce drivers v 257.21; Windows 7 x64 Home Premium

 

I could never tell a difference when I had two cards in SLI. I swapped the cards around and the labels never seemed to matter. The monitor only worked when plugged in to the top card though, I thought that was weird.