3 years is a long time in the world of PCs
A conroe chip, motherboard and half decent gfx card is going to set you back at least £450 if not significantly more
In a few months £150-200 would get you something like an AM2 board, AM2 sempron and X800 (or no doubt better by then) gfx. Just pick whatever great value components are out there at the time you buy whether they be AMD, Intel, ati or nvidia.
Put the £250-300 you saved into a high interest savings account and you could easily have a mobo, cpu and gfx card refresh every 12-18months to a dual core chip that will make the conroe look slow, dx10 gfx card etc.
ok, it may mean you are never going to have a bleeding edge system, but you'll certainly have something capable of running all games, apps etc comfortably throughout the full 3 years instead of being stuck with a lemon in your final year
When you consider you also have the resale value of the sempron, x800 etc you'd be quids in or else could probably afford something even more meaty now. There is also the fun factor of new stuff to play with
Each to their own of course - there is nothing wrong with either route. I'd just prefer to have regular "bang for buck" refreshes than paying through the nose for bleeding edge components only to find myself stuck with a machine with less processing power than a (future) toaster in 3 years
Marc