Also, you don't need to go in order with the service packs. You can do SP1 and then SP3 or SP2 then SP3, even from a gold release of XP. SP2 was cumulative and could be installed on gold or SP1 systems, SP3 required at least SP1a. So that'll save you some time.
I would also strongly recommend getting the "network professional" versions of the SP installers. These will be a couple hundred MB each, and contain everything needed to install that specific SP. No network connection needed. In fact, burning a copy of a couple of the SP installers onto a CD wouldn't be a bad idea in the event you have to do this again. You could get yourself up to at least SP3 before ever having to connect the system to the Internet, which can result in it being compromised within minutes otherwise. Might not be a bad idea to have a DVD with all the drivers you may need, a copy of the latest version of your AV program of choice, if you use a third party firewall program you can toss that on there too. Everything you need to get your system to something of a minimal baseline. You can also read up on making your own slipstreamed XP install disc so that SP3 is installed when you install Windows.