I'm not sure of processor requirements but you'll need 2gb memory for Vista -Microsoft say 1gb is enough but it's too slow for most people. With 7 you can run well with 1gb memory.
You can upgrade directly from XP to Vista but you'll need a new installation for 7.
Its requirements are even lower than Windows 7 I believe (in terms of ram at least). And, you might as well try using the OS designed for touch/tablet use?
I have Start 8 and it causes Windows 8 to behave a little erratically at times. For example, the rare times I wish to use the Windows 8 desktop and click on anything it randomly refuses to open anything.
I have Start8 on three machines and it works perfectly on all of them. I would suggest that the erratic behaviour you are experiencing is not related to Start8
There's nothing technically wrong with windows8, it's faster than windows7 and just as stable. Most people I know who've installed it have gotten used to the UI after a couple of weeks and actually prefer it in a lot of ways (although it does still have a few faults)
Installing third party programs to try and make it look and act more like windows7 just because the initial shock of the UI change is too much is the cause of a lot of problems.