I’ve always taken the requirements with a grain of salt. Officially, WinXP can run on 64MB of RAM, but the truth is that it takes up ~650MB of RAM, about ten times the minimum requirement, and thus is tricky to run on less than 1GB. I’ve done it on 768MB without flogging the hard drive for Virtual Memory, but was limited in what applications I could run without bogging; Firefox was okay, but gaming wasn’t.
There are rare occasions where the minimum specs actually have basis in reality, but given that a $100 video card is often twice as powerful as a $90 video card and ten times as powerful as a $50 card, I say that going a step beyond is worth the minimal extra cash. Think of it as future-proofing. The sweet spot for video cards in the last few years tends to be around $100–120 with a sharp drop-off in performance below $95 and diminishing returns above about $140.
Most budget gaming rigs go for an i5 over an i7 for precisely that reason too; the i7 is not always enough of an improvement to warrant the cost.
For gaming, the general rule is 30FPS as the absolute minimum which you should never go under under any circumstances, so you generally want a little extra just in case a background process decides to grab some system resources . However, anything above 60FPS is just wanking as most monitors cannot keep up since modern LCD displays usually have a refresh rate of 60Hz. Therefore, anything faster than 60 FPS is usually only good for bragging rights.
Personally, if I’m playing a game and the framerates goes over 50 FPS, I turn up the details until it gets down into the low-40s. It still looks about as smooth, but the higher detail levels look better. Conversely, if I’m dipping down below 30 FPS, I reduce detail until I get over 30 FPS as smoother motion is more of an improvement than the loss of detail is a detriment.