New System: What Sz Partition for WinXP

Hello-

As you can see from the signature my system is 3.5 yrs old and Photoshop is sluggish. Thinking of updating to: MSI 955X; 940 PentiumD @3.2Ghz, with 2 x 1 GB DDR2-800; and 2 x 500 GB SATA drives, configured in RAID1. Two questions:
1. Any comments on this system or other suggestions?
2. To setup the RAID1 correctly, I understand I should create a partition for
the WinXP OS. How much HD space does the WinXP system need to
operate efficiently and what size should this partition be?

Right now, I run a RAID1 array, but it doesn't do much good since if the OS
crashes, I need to completely reinstall the system, the drivers / applications, and the data. I believe if I had the OS on a separate partition, then if the OS crashed, I'd just have to reinstall the OS, right?

Thx.

Scott