It is very hard to say with any great level of accuracy and most of this thread is us speculating, making some theoritical statements based upon some high-level observations of the Alpha code, which is bound to be relatively unoptimised anyway. But I can say there are other people further back in this thread with weaker systems than yours saying they are running it OK in Alpha.
So I imagine that it won't run it with every GFX option enabled but it will in all likelihood run it well enough to play. And that's what I'd expect, your system is an average system for the present time and we'd expect the game to run well on it, in order there is a sufficiently large group to market to, but we'd imagine that FD would have some more graphical options up its sleeve for people with higher-end systems.
Generally it's easy to upgrade as time goes by, you have a reasonably powerful PSU which is important. I'd not bother with Win8 as you have a budget. I run Win8.1 and I have not seen a massive performance difference from the perspective of a game, than Win7 (generally Win8.1 is a lot faster for booting and other system activity, than Win7 though).
Somewhere there are posted specs for Alpha and you can check against those, I seem to remember they were pretty basic, quad core @ 2GHz for example.
Pyros
If you can upgrade your card to at least a r9 270, unless you are at sub full HD resolution.
If you are going to pay any significant amount for the OS upgrade, IMHO don't bother. Win 8.1 is indeed faster unless it requires the user to do anything on the rather messed up interface...
Thanks guys, I'm going to wait till after Christmas for better prices anyway but that's good to know that the specs are ok, the upgrade to win 8 is pretty small so I'll get that. I'm maxed out on expenditure right now, wife is watching my spending

I do plan on upgrading the graphics card though later on. Would I be right to say that I can upgrade the card to a higher one as long as I keep an eye on the psu which may need to be higher?