You'll probably want raw power and lots of memory, but it really depends on what you have now and what your budget is. The specs Valve lists are pretty much the minimum, but the best configuration (if there even is such a thing) will still take a while to render out videos at 720p.
Below are my specs. They're not the best, but loading times are limited and I don't get any lag when using the program. Rendering still takes a while depending on length and quality settings. Memory wise, you'll probably be fine with 8GB. (make sure you use 64-bit Windows, otherwise you won't be able to use all of it) I'm looking to replace my graphics card with one that has around 4GB of video ram, which should increase performance of games with big textures and long draw distances.(Skyrim, etc.) Anything in the mid range of the Nvidia 600 or Radeon 6000 series should suffice for basic SFM-ing. I list my monitor because the 1366x768 monitor requirement is really not enough if you want to use SFM. If you want to work comfortably, you'll need a monitor that is in the 1080p range. The size ofcourse depends on how far you're sitting from the screen.
[*:2z1zya40]Processor: AMD FX8350 (8-core, 4,21Ghz)[/*:m:2z1zya40]
[*:2z1zya40]Video: AMD Radeon 5870[/*:m:2z1zya40]
[*:2z1zya40]Monitor: 26" LCD at 1920x1200[/*:m:2z1zya40][/list:u:2z1zya40]
Again, this is just my configuration. If you're only now getting into Source Filmmaker I'd focus on building a PC that plays modern games like Metro Last Light or Tomb Raider at a reasonable framerate. SFM will usually perform well on those. Ask some of your tech savvy friends for help to be sure, don't blindly trust my suggestions. I'm just a stranger on the internet.