If you are planning to watch any kind of H264 HD TV, that card will be unstable. If you watch HD MPEG2 it will probably work OK. I had reliable bluray on a 3.2GHz P4 under Vista with it. Windows 7 was pretty ropey stability wise. The 10.1 driver seems to have greatly improved stability.
The AGP cards are unsupported by ATI.
I have that card, it wont need a new power supply, it just uses a floppy power connector.
It's noisy.
Check you can use a standard motherboard/PSU in that system. Dell have a habit of flipping round a few wires on the PSU, and moving the odd motherboard standoff here and there, and making the PSU a slightly different shape.