The 9500GT is weak. An HD4670 will be a nice boost, but won't require additional power.
No overclocking features make me ask, is this a pre-built PC? If so, the included PSU is probably the minimum needed for the job, and won't be able to handle a major GPU upgrade. You can pick up an Antec Earthwatts for it for $50-$60, which would allow you to put a 4830 or 4850 (or 9800GT) in it.
Be careful what CPU you get, make sure the mobo can support whatever wattage it needs. If it is only 65W, you could still put in a 5400+ for $70; that would be a big jump over the little BE.