My favorite part about my copy of WinXP Pro is that I got it for under $50 shipped to my door and it's 100% legit. BOOYAH!!!
As for whether or not to get SP1...I say why not? You JUST installed the OS, and as long as you haven't installed a lot of other things, now is the best time to do it. If on the rare chance something did go wrong it wouldn't be as inconvenient as if you had a lot of data to backup.
As for me, I updated to service pack 1 when it came out...the thing is I did a complete reformat at the time since I wanted everything to feel fresh and see what the differences are between a fresh XP and a fresh XP with SP1. I've never had any problems with this OS and likely will not upgrade for quite some time. As many times as MS has screwed up OS's (win9x anyone?) this one is incredibly stable and does everything I want it to. Oh...and even though on the rare chance something does crash(and its' ALWAYS IE) it doesn't kill the system. Of course those IE crashes are rare in XP, and most people will never see them. When IE crashed the 2nd time I switched to Mozilla. I am so happy now. Between Mozilla, Mozilla Mail, and WinXP SP1, my machine is doing everything I want it too and it's quite smooth.
I am happy.