what is your budget?

definately need a new gpu, i'd recommend an nvidia 660 as capable of running with high frame rates at high settings at 1920x1080 (£160)

i presume one 500gb drive is for data? ............ in which case i'd get a sata3 128gb ssd for system and apps, leaving the other 500gb for games (£60)

8gb of memory is plenty, you'll see no advantage from more.