I heard some people said 64 bit is not stable in many ways. Like need to find drivers for installation. and it reserves much ram for system use.etc
Most on ET are going to wholeheartedly say "W7, 64-bit" and that would be OK... should be 64-bit drivers for every peripheral. XP-64 was sort of abandoned.... works great, but you may not find drivers for all peripherals. Even if you have W7-64-bit, most of the apps you'll run will be 32-bit. Software hasn't caught up to hardware. (64-bit OS have "32-bit emulator" to run 32-bit programs. It's seamless, but not any faster.) I have both XP-32 and W7-64... functionally, I can tell no difference... so I see 64-bit as "no big deal". (I just completed a fresh OS install for my trading rig... and I did it with XP.) Yes, W7 uses more RAM, but RAM is cheap now... 4-6GB of RAM will be plenty if you shut down you rig once per day. Also... W8 is due out in 2012. XP is still supported until 2014. It would be easy to justify skipping W7 altogether and wait for W8 if you have XP discs. If you have an important app, you might check with tech support to see what they recommend.
Adding to that excellent post (I also have W7x64 but my trading systems run on XP Pro SP3, 32bit). The 64 bit os, be in 7, 8 or linux will show its main advantages handing very large amounts of data ... converting AVIs to DVDs and vice versa, image manipulation etc ... and relatively imperceptible differences for everything else.