I'm with 715 on this.
The only time I have ever truly, honestly noticed a difference in FPS between running applications in the background vs. limited processes running in a game was on PCs that could barely play the game to begin with. And it was usually on single core CPUs in non-multithreaded programs back in the day.
Unless you have a serious, out of control, poorly maintained computer, i.e. my mother-in-laws vista laptop that hunkered in 105 processes on boot-up, you probably aren't going to notice too big of a difference. You really have to have a slow computer/exuberant amount of applications running to really gain a noticeable affect off of FPS. Granted there are exceptions to the rule, that being some program dialing home or running a scan etc. These will more than likely show a hiccup.
At this moment typing this I have 54 processes running on my Win7 computer (2.8ghz C2D, 4GB Ram). If I went all nazi and turned off 20 processes, I can safely say I'd see a noticeable difference in FPS performance.
Now I'm not against keeping stuff out of the background, the 54 I have running now are programs I use fairly often that I warrant leaving them in the taskbar etc (Steam, Keyboard profiler, Gmail, AV, Mouse profiler etc). Keeping your programs limited will help your boot-up time and certain multi-tasking things, but if you consistently have to keep killing apps to keep your computer running smoothly, you may be in need of an upgrade.
I'm sure some of you will disagree with this, but in my computing experience you don't always have to go on that vegan diet.