What PSU do you have (brand/model, not just watts)?
Your CPU is two tiers below a 4850e, which bottlenecks a HD4670 at 1440x900; there's likely little point in getting anything stronger than that with your rig, or perhaps a HD5670 if you have no future upgrade plans but still want DX11.
Neither of them require a PCIE power connector, and will run even on a 300W PSU (unless it's a Bestec, in which case your mobo doesn't have long to live anyway; see the jonnyguru review "Best of the Bestec").
The HD 4670 would be more than enough. I would like to recommend the more expensive 5xxx series cards, but frankly it would be a waste of money. Start saving up for a new build and a new monitor. A great 19020x1080 gaming build and monitor can be had for as low as $1,000, and that would give you a good future proof build as well.