Ok, so you do have a pcie x16 slot then. The gt 710 will probably be your best bet then. It's semi modern (just a couple generations old), so it should be supported for quite some time. Id say you should be ok with the psu, as long as you're not doing any overclocking or adding a whole bunch of other peripherals. According to Cooler Master's power supply calculator, with everything at stock speeds, and with the gt 710, peak draw should be about 217 watts. That's a close call, but you should be ok.
edit... I put my old ati hd4670 in my dad's pc with a 300 watt power supply about 2 years ago. That card "requires" a 400 watt power supply. I've had no issues, and even stress tested the card (at 100% load) for 30 minutes without a hiccup.