A 450w PSU is more than ample for a good video card for the applications you use - providing it is a good quality PSU. What is the make and model?
Upgradevideocards.com has good information about identifying system requirements, including power, and selecting video cards, with specific recommendations for non-gaming video cards:http://www.upgradevideocards.com/criteria.htmlhttp://www.upgradevideocards.com/nongaming.html
What programs do you use for media editing? nVidia cards have special features that can substantial accelerate some video editing features in Adobe Premiere and some other applications.
Overclock.net provides a good description and benchmarks for video card acceleration in CS4.http://www.overclock.net/graphics-cards-general/389996-cuda-significantly-accelerates-photoshop-cs4-premiere.html
Adobe CS5 added acceleration for multiple applications with the Mercury Playback Engine which is designed specifically for more recent nVidia cards with the CUDA feature set.http://www.tomshardware.com/reviews/adobe-cs5-cuda-64-bit,2770.html