2 type only.
The limited account is intended for someone who should be prohibited from changing most computer settings and deleting important files.
The computer administrator account is intended for someone who can make systemwide changes to the computer, install software, and access all non-private files on the computer. Only a user with a computer administrator account has full access to other user accounts on the computer.
Members of the Administrators group have the largest amount of default permissions and the ability to change their own permissions.
2. Backup Operators
Members of the Backup Operators group can back up and restore files on the computer, regardless of any permissions that protect those files. They can also log on to and shut down the computer, but they cannot change security settings.
3. Power Users
Members of the Power Users group can create user accounts, but can modify and delete only those accounts they create. They can create local groups and remove users from local groups they have created. They can also remove users from the Power Users, Users, and Guests groups.
They cannot modify the Administrators or Backup Operators groups, nor can they take ownership of files, back up or restore directories, load or unload device drivers, or manage the security and auditing logs.
Members of the Users group can perform most common tasks, such as running applications, using local and network printers, and shutting down and locking the workstation. Users can create local groups, but can modify only the local groups that they created. Users cannot share directories or create local printers.
The Guests group allows occasional or one-time users to log on to a workstation's built-in Guest account and be granted limited abilities. Members of the Guests group can also shut down the system on a workstation.
The Replicator group supports directory replication functions. The only member of the Replicator group should be a domain user account used to log on the Replicator services of the domain controller. Do not add the user accounts of actual users to this group.
More info on Windows XP User groups at http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/lsm_local_groups.mspx
As for your questions:
1. You can convert your current Administrator user account to any of the above User Groups.
2. The ideal group to use is "Users" but note that it can only access the files that was created by this "User". You can compare which one is best or suits you by visiting the above link or http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/windows_security_default_settings.mspx
I suggest that you create a new user Account as Administrator then convert it to another User Group instead of converting your existing Admin account.
To change the account to any of the above-mentioned user groups, type CONTROL USERPASSWORDS2 or type LUSRMGR.MSC in Run box.
3. Impact - It depends on how much RAM and space you have. Depends on how Virtual Memory, Page File is configured. See RAM, Virtual Memory, Pagefile and all that stuff
BTW, if the Quoata Management is enable you can control how much disk is use by a user
Note: Quoata Management is only applicable to NTFS
Hope the above helps