Well all you'd need for the card reader I linked to or others like it is a 3.5" bay on your case for a standard floppy drive which I'd be pretty sure every case has. Still worth making sure though.
As for SLI, well I'm no expert on that myself either but as far as I know it would just be a case of a compatible mobo and nothing more. Obviously readjusting any of your previous choices to fit the new mobo if necessary but again I don't think you'd need anything more.
I couldn't say about the PSU again, but I think the 650W could be enough to accommodate for an SLI setup, but if you haven't already make sure it is a well known and reputable brand behind it to be on the safe side!
Technically I don't think MS like you using the key from your existing install on another but I don't think you can be busted for it. If you wanna play it safe and not risk that particular key, I will go ahead and wear my pirate hat when saying XP and WGA is easily bypassed if need be. Although I don't know if there'll be any measures to circumvent this with SP3, it'll be cracked in no time though!
