True.but do the games that may follow require Win8 ?
Given enough time, almost certainly yes. Practically speaking though, game developers support older OS's for a very long time. Just in the last year or so have we started to get a lot of games that require at least Windows Vista, which was released 7 years ago.
Keeping in mind that any PC bought when Vista was new would be completely incapable of running today's games due to lack of CPU and GPU power, I'd say that OS compatibility is not something that you should worry about. Windows 7 is an extremely popular OS and will be supported for years to come.
Also I see for a Radeon 7970 HD, there appear to be different manufactures or names preceding 7970.
e.g Sapphire 7970 ,XFX Twin Frozer,HIS ICQX^2 etc.
What is the difference between these cards by different manufacturers?
Is it that Radeon 7970 HD is sort of a spec/arch that different manufacturer implement differently and thus different names associated with 7970?
If that is the case which would be considered the best investment for $300+/-10
The 7970 is the actual GPU chip made by AMD. Companies like Sapphire, XFX, HIS, MSI, etc. buy the chips from AMD, put them onto boards, and attach coolers. I'm simplifying a little here because there is a lot of collaboration between AMD and their partners, but you get the general idea.
The overall performance characteristics of the graphics card are determined by the GPU chip that's on it. Board manufacturers can make small changes like overclocking the chip at the factory, but the performance difference is usually less than 10%. The main user visible differences are the cooler (read: noise and temperatures) and the warranty length.
With all that in mind, my strategy is to buy the cheapest card with a good cooler. Right now that'd be this
MSI 7970 Twin Frozr.