You've got an SFF case, so you will need a low profile gpu. This limits you to entry level GPUs, they are a little harder to find . Link below for a gpu that will work for you, you can use your current psu as the card does not require external power connectors and runs off the motherboard. I think you should likely have the 300w psu (should confirm) which will be worth free. I suggest that you update your bios to the latest version before the upgrade as well.
Then that will leave you with a monitor, get something decent, non fancy 1080p, verbally Best Buy has good options and deals, most Acer monitors come with 3 year warranty and are not bad. There are different panels, TN which Is very cheap but lacks color and is a bit harder on...