I agree with greghome don't worry about spend to much on a card as even when I make budget build pc's I have never had a problem with streaming just as long as it has the right output you should be fine.
If your streaming wirelessly 90% of potential problems would be with your internet connection.
You can get a HD5450 1GB card for £30 and it would be ok
I have in fact Radeon HD5450. I stream wireless from the PC (network N, speed varies from 130 Mbps to 200), the PS3 is wired. When playing 720p movies, it lags every 10 minutes.
Regarding the network, there's nothing else that I could do. That's the reason that I ask if I should upgrade my HD5450. I'm afraid that I pay $100 to get a better card, and see no improvement in streaming.
What do you think ?