Advice on new computer specs

[alopet]

Hello

I am new on this forum, but I have been using Virtualbox for many years. I want to buy a new computer and I want some advice. At the moment I have a quad core 2.5 ghz with 8 gig memory. I run Mint KDE 17.3 as Host. It is sufficient for normal computer work, but it is not enough to run Win7 as guest fast enough. The guest system gets 2 gig as memory. It works, but certain applications are very sluggish. Especially applications with photos and video.
Can somebody advice me on what specification I should buy for a smooth running Win7 and in some time Win10 as guest on a Linux Mint 17 or 18 as Host. Thanks very much