you could go for an AMD hudson build cpu+mobo+gpu all in one solution and add 2gb ddr3 ram. £130
OR grab h67 mobo and a i3-2100 and use its integrated graphics, and add drr3 ram. £160
OR grab cheap amd mobo and hd6450, and add ddr3 2gb. £90
Cheapest upgrade would be to just add hd6450 i guess.£40
Power wise the i3 or hudson are best option,
you could just grab a hd6450 to gain improved perfromance and keep your current rig going for bit longer then do complete new build further down the line, and carry over stuff like hdd, psu,os,.