The Adobe suite support CUDA for hardware acceleration, so that might be another reason to stick to NVIDIA. Depending on the kind of video editing you'll be doing hardware acceleration can range from a pleasant bonus to very helpful.
On the subject of video editing, again depending on what kind of video editing you'll be doing, you may find you need more RAM. Here's a 8GB kit for ~$65
NCIX.com - Buy Patriot Sector 5 PGV38G1333ELK 8GB DDR3 2X4GB DDR3-1333 CL 9-9-9-24 1.65V Dual Channel Memory Kit - Patriot - PGV38G1333ELK - in Canada
Also I would say a having another hard drive as a scratch disk is really nice to have. Maybe salvaged from an older build?