I think the rule is: all quads kinda get invalidated by the i7. Cyber powerpc offers a decent i7 rig for under 1000 dollars. You can do similar on newegg yourself. In short, if you want a quad core rig, you're being kinda dumb for getting anything less.
if you want dual, go e8500 and overclock it.
for pure gaming, dual is best, but for everything else (even just fiddling about in windows and the like) i7 seems the sweet spot. I'd also go ahead and see if you can get your hands on al ow end ssd drive just to run windows off of for that extra performance!