You can load XP on two disks but XP does not like to run with another version of itself active. Eventually something will get screwed up and you'll have corruption. One way around this would be to never have the two OS drives connected simultaneously. Unplug one drive and reboot from the other.
Honestly, there is very little reason to do what you're thinking about. A good firewall, virus software and a spyware program should provide adequate protection. There are programs you can buy that will allow you to run the browser on a virtual machine that would separate it from the rest of the system but that is kind of overkill as well.
Personally, I would load XP on the Raptor and run the Seagate as the second drive. Put the Maxtor in an external case and back up your vital data to it and then turn it off so it won't get infected.
Another way to do things i make C: a 10GB-25Gb partition and load just XP onto that. Load nothing else on there. Make the rest of the drive Partition D: and load your programs on there. EWhen that drive is all swuared away and all your drivers are loaded on D: install the Seagate and label that as E:. Store all your stuff on there. things like photos and documents etc. Don't install any programs on it.
To do this you'll have to open Disk Managemnment, R-Click on the Optical drive and change its designation to X: or Y: to get it out of the way. Otherwise it will make itself the next letter in-line.
So you'll have C: at 15GB with just the OS on it, D: wit the remaining space ofr app your programs and dribvers (things that would need to be reloaded with a reintsall of XP anyways) and E: as the onboard storage. When you reinstall XP in the future just unplug the Seagate and the data will be safe. Outside the PC you'll have the Maxtor.
This will allow you to defrag the OS quickly and keep it together, protect the system from the viruses that attack C: only and safely segregate data on a separate drive.