it's FAR better bang for buck to actually buy two pc's £1700 now and £1700 in another three years or so, else you'll have a £3500 rig that is getting beat by cheaper rigs in a couple of years time (qualitative rather than quantitative argument, can never know exactly what tomorrows hardware will deliver)
I would say get the best you can without going overkill. So an i7 and 290cf or 780sli. You would still have around £1/1.5K to keep in your pocket for any upgrades.
