I know that most of games on GOG.com support Windows XP x86 (32-bit) nearly perfectly compared to some newer systems. However, what about it's cousin - Windows XP x64 Pro? Can same support (compatibility) be expected of games to work on it? I know that those two architectures are different and Windows XP x64 Pro doesn't even support 16-bit applications, so in that effect I guess my question is sort of answered, however I still would like get further answers/ comments on this matter. Thank you in advance!

No problems running any games in Win7 x64 (I only try games where GOG state Windows 7 is supported). Remember Vista also came in 32 and 64 bit versions.

Hopefully someone else with XP x64 can say for sure.

No 64-bit version of Windows runs 16-bit applications, all GOGs are compatible with 64-bit Vista and 7 though so 64-bitness is not the problem. Windows XP x64 was never a common platform, and is often misidentified as Server 2003 (since that's what it's based on and both use kernel version 5.2 rather than XP 32's 5.1), so I'd be very surprised if GOG officially supported XP x64 Edition, but at the same time I'd be surprised if the games didn't run.

Usually there's no problem. XP 64 is not XP but Server 2003 with some features cut off, as Miaghstir pointed out, and driver support for some devices is between crap and nonexistent, but cases where it won't run something that will run on XP are few and far between.

According to this page, XP SP3 will be going the way of the dodo on April 8, 2014. Do you really want keep using it?
http://windows.microsoft.com/en-us/windows/products/lifecycle

That's 32-bit editions of XP though, not XP x64 Edition (which, by the way, only received 2 service packs, like it's brother Server 2003). If XP x64's fate is bound to that of Server 2003, I'd guess it'll get support far longer than its 32-bit cousin.

Hmm according to this, it still april 8, 2014. Unless it gets a new SP. Doubtful.

http://support.microsoft.com/lifecycle/default.aspx?LN=en-us&x=9&y=10&p1=8599

I got this old PC from a friend (5 years old) and until I get above 4 GB RAM I think I will use Win XP x64-bit Pro, until then I will use it and if it plays games well, then I don't see why not to use it xD Plus even if I have Win 7 I don't think it will take more from the old hardware that I have in that PC anyway, unless ofc I want to use DirectX11 or something and yet I only GeForce 9800GTX (512mb) anyway so the performance won't be anything impressive. Once I have 8 GB I will move to win 7 64-bit

Thanks for all the help, btw! :D

Hmm, and Server 2003 gets support only a year longer (July 14, 2015), I would've thought it'd be longer than that (given how eager corporations usually are to upgrade)... then again, it's 12 years old at that point, so it's understandable.

Keep in mind that the $40 Win8 Pro upgrade promo expires at the end of the month. If you have a non-OEM XP64 license this is a super cheap way to get a current OS with driver support for newish hardware.

I like how it's writte in dim, grey letters. Makes you feel appreciated :-P