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I was thinking more along the lines of an i3 2120/3220, its hard to find benchmarks at 1080p ultra and no aa both for cpu/gpu, but those i3,s are destroying phenoms in benchmarks, damn means almost complete upgrade.
I would sort out your processor before graphics and a cheap upgrade if your board can take one would be a phenom 965 as they go for as little as 65 quid brand new and can handle most games if you over clock them.Just ran it with firefox/apps running ....
P3393 with ATI Radeon HD 5870(1x) and AMD Phenom II X2 550
Graphics Score
4319
Physics Score
2076
Combined Score
2052
according to Result details the score is what it should be when comparing to others with same spec.
How well does your 5870 overclock and you have to take into account when those benchmarks were done the 7850 was on older drivers then, the latest drivers offer upto around %15 more performance from then plus whatever you can overclock it too i would say there is enough difference to justify an upgrade.