Having messed around with RAID a while ago when I killed mine, due to a BIOS setting, my first suggestion is proceed with caution.
You should be able to install the windows XP to the SATA drive without disconnecting the existing HD first. Only thing is Windows will insist on calling your new SATA Raid a D or other letter rather than C. When you boot from the WIN XP CD at the install options be very clear that you are getting the right drive.
If you want your new windows to be on C, then disconnect your existing HD.
If your SATA board is an older one, make sure you have the SATA RAID drivers on floppy ready to load using the F6 button during the initial pre windows CD startup.
As for size of partitions, my current Win XP only need 3.5GB, but then there is the further 1.5GB that the swap file seems to want
For real speed the swap file needs it own space where that doesn't get too fragmented. Put it this way, if your C drive, is to be Windows and a few other OS files, together with the swap file, 10GB should be adequate. Windows by default though also puts the program files, which includes some OS installed files on the C drive so you might want to play safe there and allow 16GB for the C partition. As you install other programs stick them on the D drive. I'd then have an E drive for all your data (remember you can redirect My documents and stuff to other locations).