It won't slow down your computer that much at all, unless you are creating like 30 or 40 of them. :)
If you are concerned that the limited account will effect your admin account, as far as the wallpaper goes, that will not happen. Most windows-based changes made in one account will not effect another.
A limited account does not have the ability to add or remove programs - at all. That's my experience from work, we had to get a whole new computer for this one guy Chuck because the former computer admin wouldn't release the passwords he created. (He needed a new one anyway.)
A limited account cannot access most of the higher, more important windows options (including defragmenting) except for some display changes.
A limited account can see all of the files on the computer an admin can, but one account cannot see the files of anothers account if the files are in the MY DOCUMENTS area for that user. (Which kinda stinks because we cannot fileshare each others my documents areas at work through the lan.)
Hope this was helpful! :)