This, of course, assumes you have the seventy-plus quid to spend on an upgrade, plus the resultant money required to upgrade to your hardware (especially memory), given games have higher minimum requirements when running under Vista/7, plus the huge amount of time and effort required to re-install everything after transferring all the data (and the even more time pratting around to be able to get all the older games you still play working on Vista/7).
As I have said priorly with Sword of the Stars 2 last year, there is no one game worth that amount of money and hassle. If I had £100-£130 quid to burn on a single game, I'm sure could find better uses for it.
Especially considering that XCOM doesn't even appear to have the dubious excuse that it's using Dx 10/11 as the former did.
As and when the hard-drive needs replacement, then I'll look into an upgrade, because I've at that point got to spend the time and money anyway.