well looking at the spec you could go with an i3 2100 dual core processor, h67/p67 mobo, 6850 graphics card, bequiet 530w psu and BitFenix Shinobi Gaming Case, and that would be within budget.
just over your budget (around £440-450) would be a quad core i5 2300, mobo etc as above, then for £470 ish you could go i5 2500K and z68 gigabyte ud3 mobo.
the i3 should be more than enough for diablo 3.