Is your budget that tight? If you can afford it, I would recommend the i5-2500k. Otherwise, you could go with as little as an Athlon X3 - Newegg is selling them for $80, and there is a very good chance you can unlock the 4th core. AMD vision was recommending Athlon X2 for introductory video editing, but I would shy away from something that weak.
Here is another really good option if you live near a MicroCenter
: They sell the cheapest CPU's and have a very good deal on AMD CPU/Motherboard combinations.
Yes, I was thinking of the HD Radeon 5670 when mentioning $75. If you are cash strapped, you could even go with a lot less video card like the HD 5450 for under $40. I run two monitors of of one of those just fine. AMD seems to be fairly dominant in this segment.