A capture card isn't necessary for streaming, but they do offer much better image quality compared to hardware-encoders on a graphics card. This is true for models that pass the signal through to a second PC and cards that are capable of encoding video themselves.
If you plan to record without using a separate PC, your best choices are the Avermedia Live Gamer Portable, the Hauppauge HD PVR Rocket and the Terratec Grabster Extreme HD. The downside to all of these models is that they're only capable of recording Full HD video at 30 fps. The Hauppauge HD PVR Rocket is the best and most versatile option of the three, with good image quality. We felt that the higher price was justified given the extra features, however it's rather hard to find in shops, which is why you'll probably end up choosing between the other two.
If streaming with a second PC is your thing, the Avermedia Live Gamer Extreme, Elgato HD60, Elgato HD60S and Razer Ripsaw are your best overall choices. They all have an HDMI pass-through and are capable of recording and streaming at 60 fps. In terms of software the Elgato devices are the most complete. They are relatively expensive though. In our eyes the HD60 is the better choice of the two, due to its Master Copy feature that records the stream at a much higher bit rate. For this reason it receives an Excellent Choice Award.
The Razer Ripsaw is the only capture device that supports both 60 fps and the AV input used for the Playstation 3. 60 fps is probably not that useful if you're streaming from a console, however the Ripsaw is a very useful device if you want to stream from multiple devices. It is a bit more expensive than other models however.
The Avermedia Live Gamer Extreme is one of the cheaper models in this test, but it does come with all features you need, what's more it's also capable of recording and streaming at 60 fps. It's true that its image quality isn't as good compared to the best cards, but give the price, this isn't much of an issue at all. Therefore it receives a Great Value Award.
If you're only using a single PC, the Hauppauge HD PVR 60 is the best option, since it can stream at 60 fps and record without taxing the CPU much at all.