I've owned dual CPU systems for years and always found them smooth and fast.
-> As to your comparison, see if http://www.tomshardware.com has benchmarks on your selections.
Any opinions as to which would be better, specifically for a new gaming machine? The ones I'm looking at that are roughly equivalent in pricing are the Pentium D 830 3.0GHz w/2x1MB cache vs. the AMD Athlon 64 X2 3800+ w/2x512k cache (perhaps, although a bit of a price stretch, the AMD Athlon 64 X2 4200+ w/2x512k cache). What do you guys think, and why? Also, any links to sites that compare the two?