Windows 8 is a reaction to the device-comprehensive iOS and Mac OS - from phone to tablet to laptop to desktop. All have a similar look and feel. Apple has been moving in this direction for quite some time, step by step. Microsoft tried to do it all at once to play catch-up, and has had lack-luster acceptance of Windows 8. Microsoft will keep improving it and eventually work out the issues many people have with the Win 8 UI.
Under the UI, Win 8 is much the same as Win 7. However, Microsoft often releases software with bugs or other issues, so it is best to wait a year or more to upgrade to a new Microsoft OS. I usually don't bother upgrading the OS on an existing machine except for service packs.
If you have a current PC that did not come with Windows 8, I suggest Windows 7. If you are building your own machine, again I suggest Windows 7. Windows 7 will have drivers for all current software and hardware, and manufacturers will continue to support Win7 for a few years or more. You will also have more success finding drivers for older devices for Win7 than Win8.
If you are buying a new machine, use the OS that comes with it. (Likely Windows8) The OS that comes with the machine has all the proper drivers and has been tested together with the hardware and will be supported by the manufacturer. There are ways to get around the unwanted touchscreen UI of Win8 so it feels more like Win7 as mentioned by other posters.
If you have to upgrade much of the hardware on your old machine, it is better to scrap it and get a new machine. The concept of being able to indefinitely upgrade an older machine is false. Maybe some memory or a new hard drive at some point. Much beyond that and you get into compatibility issues because your machine is too old and doesn't properly support the new gear (i.e. graphics cards, etc.) that you are trying to put into your machine. It is too costly and ultimately a poorer solution to upgrade everything.
I ultimately went and built a dedicated Windows 7 PC for my lightroom and photoshop last summer. I designed this set up just for photo editing. This is not as expensive or as difficult as you may think, and you get considerbly more machine for the dollar spent than going with a Dell, HP, etc. ( If you want the components list I used, message me directly. I did a lot of research prior to building out the machine.)
I use a laptop for my daily use machine. I use dropbox for the occasional file swapping between machines.
XP is dead. Most anything you buy today - either software or hardware - will no longer support XP. It is definitely time to upgrade.