XP runs art programmes faster and more efficiently than the bloated cash-grabbing whore that is Vista, so I wouldn't worry about that. If your PC is getting on a bit, it might be an idea to assign your tablet as the only monitor rather than running it and a regular monitor concurrently as an extended desktop, as this puts more of a strain on your graphics card (not much, but if you have other things going on it all adds up over time, especially on older machines).
Manga Studio has some great features, but I would stress that even coming into it with previous experience of art programmes, there's a lot of it that's counter-intuitive and obfuscated to the point of practical non-existence, including some quirks like not being able to assign functions - like zoom - to your Cintiq buttons (or if there's a way to do this, it's well-hidden enough that several years in I still haven't found it), and it took me ages to figure out that you can't open files directly in MS, you have to open a document with exactly the same dimensions and then import the file as a separate layer (if there's a way around this, I haven't found that, either), plus I still haven't figured out a way to make MS remember where all the different tool palettes are located when I restart, so it randomly dumps them in the centre of the Cintiq after I turn it on regardless of how many times I try to save the workspace. None of these are deal-breakers, but they are annoying.
The stroke in/stroke out correction options are great for doodling, mind, though I wish the same could be said for the line tool, which tends to be more jagged along the edges than I'd like. File sizes tend to be quite large, too, but I've had less trouble with MS than I've had with larger files in Photoshop.
