Need to format, which Windows version should I pick?

My Dell e1505 was never released with any OS newer than Windows Vista but I was still able to install Windows 7 and get everything to work properly even though Dell has no Windows 7 drivers for it. For the most part each release of Windows contains drivers for hardware that ran on prior versions even for systems that weren't originally released with that OS. You can also use Windows Vista drivers in Windows 7 in some cases. I see no reason why Windows 8 wouldn't work either.

Uhmm pretty much all works the same dude, 32 or 64 bit is all you should be concerned with when getting an OS.

Vista with SP2. And contact their support and check if you can upgrade to 7 Pro or Ultimate. If it came with Vista, i doubt that your hardware could go any higher than 7 and even 7 might be a stretch.

You're probably better off just getting a totally new PC, one that is Windows 7 friendly. Win Vista is horribly buggy, between word of mouth and what limited experience I've had with it. Windows 8.0 isn't much better than Vista either (although I hear 8.1 is a vast improvement over 8.0).