The 7750 is the fastest gpu you can buy that still runs only on the 75w power of the PCIe lane. Your psu is rated at 300w, but I think that is optimistic. Since you already have a card that runs 75w or less, I picked the 7750 as you made no mention of changing the psu. If you can spend ~$60 for a bronze certified rosewill, corsair, antec or seasonic psu, you can get up about 500w true power with enough grunt to run a good midrange card like 7850/70 GTX660Ti, etc.
If you want to game, you MUST have a quality power supply.