I'm going to recommend against the RAID 1 configuration. The performance gains for gaming are very small to nonexistent, and you're looking at a large increase to your chances of failure in exchange for this small performance gain. Just one opinion, take that for what it's worth.
One thing I really like, is a dual boot. I share the computer with the "mrs." also, and having that dual boot option makes it more or less like having two computers in one box. She's free to do what she wants with desktop settings, shortcuts, startup programs... whatever, without affecting me in any way, and vice versa. Well, well worth the $100 in my circumstance. I don't know if that sounds like something appealing to you or not, but I thought I would make mention.
OK. 20" screen, I assume it's a 1680*1050 resolution.
Dual boot would run you about $125 more or so, and would be pushing that $2500 budget, so I left it at a single OS. I also did not select RAID, but this is easily changed if you're very firm on that point.
Hope you find this helpful.
Edited by Tyler Lowe - 06 Sep 2008 at 11:03pm