need some advice :D

You need to know what graphics bus you have. Typically you would have either an AGP or a PCI Express bus. An easy way to figure out which one you have without prying open your case is to download and run CPU-Z and then go to the mainboard tab and it will be listed under Graphic Interface.

Here's a good site for figuring out the best card for your money: Best Graphics Cards for the Money review

Basically get a Radeon HD as they will do what you want for the cheapest amount. You want one that at least has hardware h.264 decoding for watching high-def movies which means get at least a Radeon HD 4xxx or a Geforce 8xxx or better cards. Also if you get a card with HDMI most of them should output both audio and video which you can connect to a receiver or high-def TV.

Get this one if you have PCI Express.
Get this one if you have AGP.

But honestly though $50 isn't going to get you a good enough card to play the latest games with. Your going to have to spend more for that. And I'd only get a gaming card if you did have PCI Express as AGP is not going to keep up. If you do decide to go that route I would suggest getting at least the 5750 if you are real tight on money but the 5770 would be a lot better as it is further up the hierarchy chart.

edit: I did find a really good deal on Newegg for a Nvidia GT 220 as an Open Box. Its probably the best deal you can get with a PCI express for your price range. It can also play all the games available but you will probably have to set them to low to be able to play. Here it is: http://www.newegg.com/Product/Product.aspx?Item=N82E16814121347R If you look at it on the hierarchy chart its a few rungs above the 4650 I recommended.

Check out my PC Tips in my signature as well as it covers how to run today's games on low-mid range computers.

I echo what Nachbar said but would be helpful to know a little more about your system. Processor speed, memory, etc. Even a new video card may not improve your performance if it is bottle-necked by the rest of your system. Definitely run CPU-Z it's great for getting details on your system.

totally agree with the guy up there, sometimes the problem is the rest of the computer.

A graphics card for under 50 bucks is not going to be good for anything but solitaire...you wont be doing any "gaming" on it. I would save up for a better computer if you are looking for a graphics card in that price range.