I was just looking at your computer specs in your profile - and you wrote that you have 168MB of RAM, I dont know how thats even possible...
But, if you have 256MB of RAM or less, you should really be considering upgrading your RAM and not the graphics card, and you have a hdd for videoediting - you would really need to upgrade your RAM, you will get a good boost with video editing, than with a new card.
About the AGP vs PCI-E, depends when you are going to buy a new computer...
There are still a lot of AGP cards to buy, but both ATi and nVidia are switching to PCI-E, leaving only low and some mid-range cards available for the AGP.
Really depends how much you're playing new games - for the newest ones you have to upgrade to PCI-E, cause thats the only way to get the high end cards, but I think AGP will hold on for another year or perhaps even two before it becomes abandoned - in that case you could buy a decent AGP card to help you get over these 1-2 years (for the 2y period I'm recommending 6800GT, it aint cheap, but you will be able to enable PS 3.0 in games).