Both builds will be capable of running all modern games at varying graphics settings. The only games that will tax the 4870 are crysis, empire TW, GTA 4 (more CPU bound though) and possibly Far Cry 2. You dont mention your monitor resolution, but at 1600x1050 all games will be playable.
In terms of future proofing the system, i think its a tough call. Niether chip is high-end, but both are very capable. The Intel chip will overclock higher, whilst the AMD chip has the advantage of an extra core, which will come in handy should you multitask a lot, or video encode.
Crossfired 4770's will outperform a single 4870, but your original budget does not stretch enough for 2 4770's. I have no idea of what money you will have available in the future, and so can only advise on what you have said. If your gaming resolution is lower than 1600x1050 i'd go the 4870 route. If higher, then the 4770 route, and add another at a later date. I only say this due to the extra vRAM you'll have from 2 cards. If you can see yourself having more cash in the next 6 months or so, then the price of the 4870 will fall furthur still, enabling a nice crossfire setup. To make a worthwhile upgrade from 2 x 4770's, you'd need to replace both cards, rather than one from the 4870, which will last you fine for 6 months assuming your not gaming on a 22'' + monitor. The Biostar can also accept Intels quad-core chips should you wish to upgrade at a later date.
You have to remember that you are wanting a high-end gaming setup for £400, which simply doesnt happen. An i7 motherboard can cost in excess of £300 on its own, so unless you can afford to spend pretty much double your budget, then you are at the mercy of the fast-paced pc world. Both suggested setups represent the very best you'll get for £400, and both setups will run the latest games, and, with some cheap(ish) upgrades, will continue to do so for the next year or so.
I'd go the Intel route, get the 4870, and in 6 months time upgrade the CPU to a quad, which will be cheap by then, and either crossfire another 4870 (if low res) or sell it and buy a new card (58xx series or GTX 3xx series) if you game at a high resolution, or can't live without AA in games.