quad core processor to a dual core processor...

[Hitman Smurf]

If you already have the processor why change?

[Luggage]

You get most bang for buck on dual video cards if you have a huge monitor (>>20 inch); otherwise I often find the single cards to have better performance; of course, if you get an awesome deal on a second video card that's another thing.  You should check out tomshardware.com where they do a very nice comparison of FPS for any video card you can imagine vs. all sorts of dual and triple card setups

That said, I totallly agree with Hitman.  Only get a video card if your frame rate is well under 60 on average for the games you are playing.  If you are playing CS:S, TF2, etc. (not sure about L4D2) then you really don't need a second card -- will not enjoy any meaningful benefit with second card.

Btw, I would say the same thing about quad-core processors.  For these games I can't imagine the processor being a bottle-neck although I do think a dual core will kick the quads ass in gaming if you spend relatively similar $ on each.

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