Quite apart from the voracious appetite that vista has for hardware, and leaving aside the DRM issues (some FUD admittedly), there are a lot of changes in the way that drivers and software interact with the OS.
As a result, large chunks of existing software won't run, and large numbers of peripherals won't work.
The former is largely down to the lazy programming assumption that "everyone has admin rights". Not the case under vista.
While MS has doubtless invited hardware manfrs. to write drivers for their peripherals to work with vista's increased security (much less access to protected kernel mode), many manfrs. will understandably be somewhat tardy in supplying drivers (let alone certified drivers) for their older products.
This is quite different to the situation with Win2K and XP, where drivers were frequently interchangable. It's more like the leap from Win 9x to XP.
So unless you upgrade your hardware to run vista, then upgrade your software to work on vista, then replace your older peripherals to work with vista, you would be better off sticking with your existing OS.
Or trying Linux
SD