The GMA 3100 should be fine for watching 1080p video... at least that's what my Google search tells me...
However, for playing games especially at 1080p you want something more powerful than the Radeon HD 5450. Assuming you have a decent 300w power supply, I recommend a Radeon HD 7750. It does not use much power (47w peak at stock speed) and it is actually pretty powerful all things considered. It is as powerful as the older Radeon HD 5770 and it also consumes about 45% less power than the HD 5770 which makes it a great choice for a 300w PSU.
The issue however is your case if you have a "standard size" case then you can buy the following Radeon HD 7750 for $70 after a $30 rebate. However, I personally prefer the Radeon HD 7750 in the second link for $80 after a $20 rebate because the hot air is exhausted out of the case rather than simply dumped inside the case.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131461
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150663
However, if you have a "low profile" case which cannot fit a standard height card (5.5 inches I think), then you will need a low profile version of the Radeon HD 7750 for $100 after a $10 rebate.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202002