I have linked the Newegg.com store section with "low profile" cards. Typically, only low profile cards will fit in a slimline desktop. http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010380048%201069633099%201419531894&bop=And&ActiveSearchResult=True&Order=RATING
Depending on how much you want to spend/how powerful you need it to be these are your best options (from lowest price to highest) Make sure the listing says HDMI--most have it but not all.
HD 4350($19 after rebate), 4550 or 4650, 9600GT, 9800 GT($99)
The amount of power you need will depend on the size and resolution of your screen, and if you want to do more than just watch video. The 4650, 9600GT, and 9800GT would be powerful enough to do some average gaming.