New Graphics Card

I would go with a GTX 650 ti, powerful but cheap  

Hi. You can get the HD 7870 or GTX 660 for $195 after rebate. Or if that's a little too much, then get the HD 7850 2GB. But if you prefer Nvidia, then the GTX 650 Ti.

Will my PSU be able to handle these?

Your PSU can handle all the cards mentioned above.

Great! Thank you all for your help. Looks like I'll start saving for the GTX 660.
What website has that rebate offer you mentioned?

Newegg.

Sometimes the GTX 660 can be had for even less like $180 a few days ago. It looks like it's out of stock for the moment.

ZOTAC ZT-60901-10M GeForce GTX 660 2GB
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Since you're not in a rush to buy, you can keep an eye out for one when it goes on sale for less.

Does it matter which company I buy from? I was thinking EVGA, because that's what I have now, but will there be a difference in performance?

different brands have different level of factory overclocks on their cards so it depends. though zotac amp edition usually have the highest factory overclock on their cards and are also the most cool and quiet but are usually priced a little higher than other brands like evga. however evga have better warranty than zotac so you must decide what you want. if you want the best price and best warranty then go for evga or if you want the best performance and best cooling go for zotac amp! edition