A cloud service will not work for FPSC. I'm not sure what compression algorithm OnLive uses (some say it's custom, some say it's standard, I imagine it's probably a bit of both), but it requires a hell of a lot of power to render, compress and pipe the frames to your computer. AFAIK there aren't any generic cloud offerings like this (your best bet would be renting a BitCoin miner, or finding a render farm as you mention - but it's not going to be cheap at all, and I very much doubt they'll give you OS / hardware level access).
You have a perfectly decent computer already. The cost of a Windows license (if you don't already have one) is <£100. Bootcamp or virtualization are the two cheapest options available to you - they will also work a damn sight better than cloud options.
Which model do you have? I believe any recent Mac has a decent (enough) graphics card in it.
