There are additional updates not previously released. Go thru windows update and do the express install, it ill only upgrade those components you need. Its about 35 to 40meg download on an up-to-date computer, more on one that hasn't had any patches installed.
So yes, even if you updated to right before SP1 was released, you gotta go install it anyways.
Side note: The new XP NVida drivers totally kick ass. I gained an avg of 40fps!
[url:8b790]http://www.nvidia.com/view.asp?IO=winxp-2k_40.41[/url:8b790]
Also get the 40.41 hidden features patch here:
http://www.guru3d.com/files/