the GTS 450 will run most online games, but the 550 may have some advantages for your video editing needs.
but i suggest getting the GTX 460 instead as it will outperform both. take note that there are two versions of GTX 460 the 768MB and the 1GB. i suggest go for the 1GB.
there are also better alternatives such as the 6850, but i don't know if you're software on video editing would run better with an nvidia card so stick with those choices. i suggest getting THIS GTX 460 though: