Hi,
What about GTX 1050Ti? It will bring huge performance boost over r7 240 and it is written that the recommended PSU for 1050Ti is exactly at 300 watts, just like yours. I think 1050Ti is the best option since more powerful GPUs will need more power and better CPU, which leads to building a completely new system. I think GTX 1050Ti is a solid variant, even a non Ti version would be a great upgrade.