My experience in win XP was that it performed quite well, for some games ns included.
I actually miss my second monitor witha passion right now. I will be investing in 2 new monitors soon I just dont know what ones yet.
The problems:
cant access second screen while playing first screen full screen game with out minimized, might work if you run in windowed mode though i never tried that.
Games sometimes center themseslves and tehn maximize casueing teh second screen to do the same, examples of this included c and c generals.
some games will let teh cursor leave the game area and travel over to the second monitor causing a focus shift and wondows willl minimimze the game. again cc generals.
i really like having all my irc aim msn crap in one monitor, but it can be a pain, thouhg for certain games you simply have to turn the second screen off.
i also think that windows media player 9 supports the second screen.
you can get two keyboards like i have and problably two mice as well but you need usb and it still will run on the windows focus system so it wont help you.
if you want that i suggest going out and buying a crapp old machin and running a linux install on it on so you can send ims read irc on one screen and on teh other play games like a manic. in only would go with linux on teh second one because it will allow you to run an utra craptastic machine.
if you want a huge desktop you can but monitors with insane aspect ratios an example being the mac studio displays. i know they are macs but they are quite nice:
http://www.apple.com/displays/ and witha native 1680x1050 resolution how can you go wrong on the 30 inch ones it hits 2560x1680 for utter insantity. in fact i need to buy a video card taht can handle two of these babies:)