Here's my experience with 64-bit OSes. Hopefully this will clarify some things that have been erroneously mentioned in this thread. I've been using XP x64 for 3 years, and W7 Ultimate x64 (RC and now retail) since the RC was available. So I've suffered every possible problem (except for viruses and trojans). The biggest problem I've had has been with non-mainstream hardware devices. So my advice is stick with the big players, and if your hardware is less than 18 months old, you'll have a much better chance of finding drivers. Even then, you may have to dig around on the manufacturers' site a bit to find what you're after. It takes about 20 clicks to download my Canoscan 5200f 64-bit drivers from the Canon support site, for example - and I know how to get there! The other problem is Adobe. They haven't provided any 64-bit support for any of their current apps, including Flash. Silverlight might beat them to it, in which case Flash might be a thing of the past (hooray!). But 32-bit browsers will run fine on x64 and display flash if that's what rings your bell. Personally, I just prefer using the 64-bit browsers, if a site absolutely requires Flash features in order to navigate and view the site, it's not worth my viewing it. That's what HTML is for, IMHO. There is precious little software that WON'T run under x64 natively (i.e. not in a virtual machine). Most of those are due to the old installers not running under x64, but if the developers have released newer installers you'll probably find nearly all of the "incompatible" software isn't actually incompatible after all. Some glaring examples are many Ubisoft game titles, which install fine but won't execute, and of course copy-protected CDROM games and apps needing special dongles, so ymmv. For software that ONLY runs under a 32-bit OS, you still have three alternatives running W7 x64. They are: 1) Virtual XP mode (free, but only runs under W7), 2) MS Virtual PC 2007 (free and runs on all MS OSes), and 3) VMWare (also free). I stopped using VMWare years ago, they're moving away from the free model and moving into the corporate server-farm business, which means their support is pretty damn bare, and the hideous complexity of configuring and maintaining their VMs meant I was spending more time managing and less time running my software. If you want to try these options out, try downloading and installing the Virtual XP Mode package and try that first. It didn't work for me because it's significantly cut-down in terms of hardware support (particularly the horrible floppy support), and it precludes using the MS Virtual PC on the same machine - you can use one or the other, but not both. VPC might be a better alternative for many folks who need 32-bit OS capability - you have heaps more control over machine configuration, it still has floppy and CD support, it supports NAT, and it's much simpler to control. And it's still free! I'm running Windows 3.11, W95, W98, NT 4.0, and Windows XP simultaneously, so it's definitely more flexible than Virtual XP mode. Again, YMMV! Security... Well, there's a loaded word! Let me put it this way : I was involved with multinational corporate network security for 7 years, and I use a dedicated firewall PC running Windows Firewall (XP 32), and I don't have ANY antivirus or any other firewall software on either machine. And I've never (EVER!) been infected with anything. That's probably due mainly to the way I use and interact with my email client (Outlook) and websites generally, rather than any inherent "extra security" offered by the x64 OS. Having said that... Most binary virus and trojan code won't execute correctly in an x64 environment without throwing errors (this includes buffer-overrun type attacks). I do have DEP (data execution prevention) turned on, but I've never had any binaries that trigger that protection mechanism (apart from some older Adobe products...there's that name again!). In terms of speed, there's generally little improvement running the same app, configured the same way, on the same hardware, between a 32-bit OS (XP or W7) and a 64-bit OS (XP or W7), regardless of the app type. 64-bit OSes seem faster on my system as they have access to memory past 3G and use 64-bit PCI transfers for my 64-bit I/O (Lynx L22 and 3Ware 9550 RAID), but that's about the limit of the improvement. All the speed benefits have come from improved and optimised I/O configuration (PCI latency works differently on 64-bit PCI buses when the OS is also 64-bit, and other BIOS settings will have an impact, depending on what settings are available in your BIOS). Since I deal with extremely large audio (up to 16Gb of 192k/24 bit stereo per file) and video (up to 400G uncompressed video), the throughput improvement is significant compared with XP32, but the apps run at roughly the same speed, just loading and saving (and streaming) is faster. But that's enough to save me 25 minutes of thumb-twiddling every day. However, other users have posted here that they see up to a 30% speed improvement when running x64, so YMMV. Remember, too, that developers will tend to optimise native 64-bit applications, so the coding improves the speed, not necessarily just the underlying OS. But in every case, given the choice between a 32-bit and 64-bit app, I'll take the 64-bit every time. It's just Adobe who don't give us that option - my last support call with them ended with them recommending that I uninstall W7 and install XP32! I'm so sorry for the lenght of the post, congratulations if you made it through in one go . And I hope this helps clarify some issues and helps with choosing the best OS.