then you'd be better off saving up some money(5-6k)
and change your proccy(e5400@3k or e7500@5k) and upgrade your ram(another 2gb ddr2@2k)
and let your graphics card fly!!

this setup will surely take care of your performance problems
don't sell your graphics card
'cause you wont find many buyers at that price
gts 250 isn't far better than 4850
it's just the same performance category