I agree with Raze that's a good place to start. There's also
www.notebookcheck.net its available in multiple languages and has the largest collection of notebook benchmarks on the internet; and
Notebook Review Forums for asking questions.
XP is being phased out in favor of Windows 7. There are still some compatibility issues with very old games, however many new and upcoming games take advantage of the newer graphics features in Windows 7 (Direct X) that will never become available to XP users.
If you're looking for a gaming notebook you should prioritize what features are important to you besides the graphics card and screen size, such as is color accuracy or brightness important while gaming or not gaming (such as watching a HD movie), ports such as USB and eSATA if you have external storage connected to your notebook, etc. If the only primary factor is good gaming performance I would find the best graphics adapter with a keyboard layout that works for and the 17" screen.
Personally I've found the ASUS G73Jh and G73Jw (depending on your preference for ATI or nVIDIA) to be a great value. Both of these notebooks are often found demoing Crysis on high in big box stores; this answers the question to greatest running gag in gaming for the past few years. The keyboard layout and fan placement (exhaust in the back) is inline with most high end gaming notebooks, but both of these units sell for a fraction of the cost of an Alienware, Eurocom, or Sager.
Before you make you decision I suggest checking out the links everyone has provided and also consulting
Windows 7 Software Compatibility List to ensure the games you want will be playable on Windows 7.
Note: many of the new gaming notebooks can be installed with XP, but you'll need to hunt for the drivers yourself on the internet since the manufacturer does not provide or support them.
