Just got my Alienware X51 last week, and was surprised at how low the Windows Experience Index score was.  It was the same as my work laptop, and that thing is definitely not designed for performance... so I decided to do something about it.  I replaced the 1TB Seagate HDD with a 120GB OCZ Vertex3 SSD, and did a fresh Win7 install from the provided DVD.  Now check out the result:

I used this installation and tweaking guide: http://www.overclock.net/t/1156654/seans-windows-7-install-optimization-guide-for-ssds-hdds to make sure I set the options correctly.

Since there isn't much information out there about performing the upgrade, I figured I could answer some questions about what I did, and maybe we can learn from each other here.

If there is enough interest, I may open the case again and snap a couple of photos to show how I installed it - it did require a non-standard solution to make the SSD fit properly, since the SATA plug and tray did not line up exactly.