I also have a Corsair 450w PSU, the HX450, it's now in my Media Center, with the card that also used to be in my main rig, a GTX460 1GB. In terms of TDP, that's as far as I'd go for precaution, considering the rest of the system and a bit of overclocking headroom.
The GTX460 1GB has a 160w TDP; you can get better performance from a Radeon HD6870, which has a 151w TDP. At the monitor resolution he's going to be playing, a Radeon HD6870 should be more than enough for almost anything.
If you want to go with Nvidia, there is also the GTX560, which has a 180w TDP, it should do fine, but I don't know if that is not pushing it too much if you consider overclocking the CPU and / or GPU. Either way, a GTX560 for a 19' screen with that resolution is overkill at the moment. A GTX460 1GB will do fine too.
Edited by tpi2007 - 5/31/11 at 7:38pm