Which VSL string libraries do you want to run on your Windows machine? And what other libraries are you going to be using on it?
Special Edition libraries are the least "system taxing" then, VI string libraries (much more samples and articualtions then Special Edition). Dimension and Syncron libraries require a lot more resources.
Syncron libraries have "baked in" reverb. All of the other libraries are completely dry, so if you are using them you will probably want to use MIRx (if you are only using VSL Special Edition or VI libraries because it can only be used on them) or MIR Pro or Mir Pro 24 if you want to bring in other libraries and audio into your virtual concert hall.
You'll probably want to run Vienna Ensamble Pro on your Windows slave, and I'd highly recommend getting Vienna Instruments Pro as well (unless you are going to use the Syncron libraries in which case you won't need it).
If you tell us more specifics about about the type of string music you will be using this machine for, I'm sure other members who know a lot more about hardware then I do will be able to help you out. For instance if you are writing string quartets with Sole Strings 1 you won't need near the computing horse power you will need if you are using a Dimension Strings library that can have 24 individual instruments playing at once.
Unless you already know that you are going to go with Syncron Strings, an easy way to familiarze yourself with MIR is to buy a single VI instrument such as Solo Violin 2 or Solo Celloe 2 with them you will also get the standard versions of Vienna Instruments and Vienna Ensamble and you will be able to demo Vienna Instruments Pro, MIR Pro, and and Vienna Suit (awesome set of FX) for free, not sure if there is a free demo of Ensamble Pro right now, I'm sure someone from VSL will chime in.
With the current sale it's a good time to buy. Hope this helps and welcome to VSL!
Michael