Not sure which Windows to get.

The difference in the Interface of Win8 Vs Win7 not that it will 'improve the performance' of your applications, it won't. What it can do, which many of us dislike, is become more of a headache for Keyboard / Mouse use since the UI is based on using a 'touchscreen'. Some people really can adjust to this (I dunno how) and can't see living without Win8 interface, I personally gave it several tries and find it is too cumbersome for K/M.

If you want improvement for your software, you need to center around the GPU and Memory, on the given your not on a 'generic' HDD of 5400RPM. Both will improve time and speed of editing, but still rely on the 'slowness' of the HDD to read off of. Which is why most people look to SSD + HDD mulitple drives, putting the software on the SSD to boost the 'feel' and speed but HDD to do the storage.
 



This is what I'm looking at:
Intel Core i7 4800MQ Quad Core Processor 2.70 GHz (3.7GHz Turbo)
Arctic Silver 5 Thermal Paste
16GB PC3-12800 1600MHz SO-DIMM DDR3 Memory (2 x 8GB sticks)
120GB Intel 525 Series mSATA Solid State Drive
250GB Samsung 840 EVO Solid State Drive
Notebook BD-Writer (Playback Software required)
Standard Onboard High Definition Audio
Windows 8.1 Pro 64 bit
I have a 1tb external hard drive as well.