What PSU do you have? GTS 250 and HD 4850 are similar in performance. They are better than GT 240 and HD 5670, but consume more power (so you would have to have an additional power connector from your PSU) and are a bit more expensive. I would suggest the GTS 250, but only if you have an additional power connector and the required money. The next recommendation would be the HD 4850 > HD 5670 > GT 240.