Minimum specs for a 1080p VPX would be something like
i5 quad with the highest clock speed you can find
4GB RAM, 8GB if you can
GTX 760Ti minimum
HDD/SSD isn't going to make any difference to the in game performance, but will make the thing run nicer all round generally - things will load quicker.
Depending what is currently in the box you may be able to just swap the card and CPU out.
However, first thing I would do is install VPX and give it a go! - That way you'll know what's what and it'll cost you nothing!