you don't have many options... with a 300w power supply, and it has no PCI-E power cables... the GTX750 is $100ish after rebate and doesn't need power cables or use much juice. I'd say it's about your only option.
what are you getting for $100.... well lets put it this way. your computer Radeon 7450 card scored 323 in G3d benchmark test. the GTX750 scores 3,341. so literally 10x as powerful
you don't need to upgrade the PSU, if you don't overclock... your system draw at full load would only be about 200watt