Ok guys I made the switch, I'm currently using the 120 days trial, and I will receive my retail CD-Rom by mail next week. I re-installed my Vista x64 on a separate HDD, and now the Windows XP x64 on another one (I got three HDD total, only two at a time used in the system). I've installed everything so far without a single problem since I had all the required drivers, and to my surprise lots of old 32-Bit programs are installing just fine (I say to my surprise because those same programs including one game just refused to install on my Vista x64 partition).
The only problem I have so far is that sometimes when I boot up right after the Windows loading bar finishes then the screen remains black, but my monitor stays open (green light), it stays like that, idle, doing nothing (no HDD activity) for around two minutes, and then suddenly it resumes the loading process up to the "Welcome" screen, and then again stays there, idle at the Welcome screen doing nothing for another two minutes or so, and then it suddenly resumes to the Desktop. Then when the Desktop finally loads then I notice that many programs and Windows maintenance tools (Management tools like the Disk Management for example) are taking FOREVER to respond (like two minutes each or so)...
BUT... sometimes it just doesn't do it at all, and I actually know in advance when it's going to do it or not, simply because if the Windows loading bar loads for too long (like when it does for example 10+ "passes") then it means that it will have problems to respond. However if the loading bar does around 6 or 7 passes and I see my keyboard F-Locks light flash immediately then I know it will work properly and the Desktop will be reached in a matter of seconds, as usual. That problem I'm describing is also occurring with my Vista x64 installation. I've NEVER ever experienced such a problem before on my x86 partition of XP, so it's probably something to do with x64... or some of my hardware, but then if hardware is the cause then why as I said my XP x86 installation never did that ever, I don't really get it.
But really other than that I have no issues at least not so far. I've only installed my Steam and all the games I have on it (Orange Box and many others, around 35 in total) and they all worked (well, "worked" I'm still not 100% sure but they all launched without problems, which means they should be working right). So, as of now I'm quite happy with it. I'm just trying to leave the 32-Bit architecture behind and move to 64-Bit permanently, but I love XP, and I didn't feel comfortable to just abandon it for Vista since I'm still not quite used to Vista for now (the GUI and the very annoying - although practical - UAC prompts). As long as I can switch between XP x64 and Vista x64 (usually I go with Vista x64 with DX10 gaming and nothing else really) then I'm alright with that, I just didn't want to get "rid of" XP forever, it's quite simply the best OS I've ever used to date (I've had Linux for some time, along with previous Windows versions, and XP is definitely my favorite for its simplicity even if it's "full of security holes").
Vive Windows XP!