That will definitely help HD playback since the new graphics card will have much better hardware acceleration for that type of content.
Much better for some light gaming as well.
Don't worry about your power supply, that should be able to handle that card without a problem.
it will help the netflix streaming, but remember for 1080p streaming you are going to need one hell of internet connection (15mbps down 5mbps up, preferably fios or other fiber internet with 20 down 10 up, the up doesn't matter as much though)
I'm using an older ATI HD2600Pro and not experiencing any issues with Netflix Instant Watch. If your current graphics solution isn't cutting it for you (and you're not experiencing slow internet download speeds), then I'd probably recommend an ATI HD3450.
Also, I don't think Netflix offers high definition movies over Instant Watch. They only mention DVD quality.
From Netflix Q&A:
Q: How does my Internet connection affect picture quality of movies I watch instantly?
To watch movies instantly via your PC or Netflix Ready device, we recommend a broadband connection providing at least 700 Kbps . The faster your connection, the better the quality. For DVD quality video and audio, you need a connection providing at least 2.4 Mbps.
Your connection speed will rise and fall depending on network usage in your home and conditions at your Internet Service Provider.
I have my toshiba satellite l45 something or other POS laptop i got 2 years ago and its connected to my 42inch LCD. i stream netflix on that with a 10mb down connection. I have no problems at all. The video is fine..its not blue ray or anything but, its quality picture. Heh, ived played Counterstrike 1.6 on the laptop before...it was a bit choppy but playable...