I think first off we need to know what resolution you plan to play at, as that's an important thing to know when considering price/performance.
It'd also be nice to know what sort of system you're coming from.
Assuming you're coming from something old and terrible like a P4, obviously getting a dual core is the best bet. In all honesty at the moment you really aren't going to see that much in the way of noticeable gains for most games on a quad over a dual.
If you're dead set on Intel, for the best deal I'd be considering getting either an e5200 or an e8400 or something similar. Both should be more than adequate for today's RTSs and FPSs, especially if you overclock.
If you'd prefer AMD, there's nothing wrong with one of the new Kumas, like an x2 7750 (or if you want something really cheap, an x2 5200 is about $60)
As far as the mobo is concerned, if you're going AMD with a dual core definitely get an AM2+ that supports the new P2s for future upgradeability. As for upgradeability and good price/performance for Intel.. well sadly that's kind of hard considering the new socket and all.
Now, if you lied when you said you wanted the best price/performance and you actually want a quad core, I'd suggest a q9400 over an i7 any day, if we're still sticking to p/p in that segment. As for AMD, a P2 940 is comparable in p/p.
In regards to video cards, I'd probably get an HD 4850 for the best P/P, or if you wanted to spend even less an HD 4830 or a 9800 GT are both quite capable.
RAM: I'd stay away from DDR3 right now, as DDR2 is _really_ cheap.
I think the easiest way of cutting costs is crossing the OS and Office off your list. I have a friend named Mr. B.T. Rent who might be able to help you.
However, my post is probably entirely useless to you as you mentioned buying a $1700 system with an i7 earlier. Oh well.