Honestly you won't see a major difference in having 4 cores even on FarCry2. I think in 2 years as you've said maybe some programs will actually use 4 cores to its potential where 2 cores are not able to catch up. As of now and probably 1-2 yrs later it won't matter that much. I'd save my money if I were you and upgrade when there is a need to since you probably won't be sticking to your current specs by then anyways.
A Dual@ 3.8 in non-multithreaded games can't really be beat right now. If you were playing FSX or doing video encoding it would be a different story, but I don't think it's worth it, unless you want a quad for background tasks while gaming (anti-virus, encoding, whatever).
Sorry for the double-post, but it won't let me edit.
Consider changing out your 3870s for an HD 4870 or a pair of 4850s, it's around the same price as those quads (~$250 and $320, respectively), but will have a significant impact on your gaming preformance (depending on your resolution and detail levels).
I agree with upgrading the video cards first. Your CPU is adequate at that clock speed to handle just about any current gaming title you can throw at it.
I would avoid the Q8200 as a gaming CPU altogether. It has a low multiplier (x7) which is going to be somewhat difficult to get high above 3GHz with DDR2-800 that you have. Aside from that it has a low 4 MB shared cache and the games that you've mentioned are pretty cache-happy. You will likely see a huge dip in performance with this option.
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