I say switch to a core 2 duo simply because there are no games made yet that will use the full potential of the Quad core. The core 2 duo has a higher FSB and faster speed. My gaming friends all use the core 2 duo.
This is not true. Many games take advantage of quad core CPUs. Some popular examples are Supreme Commander, Lost Planet, Hellgate: London, Splinter Cell: Double Agent, all Source-based games (Half-Life 2 series, CounterStrike: Source, etc.), all CryEngine2-based games (just Crysis right now), and all Unreal 3-based games (Unreal Tournament 3, Gears of War, Mass Effect, and many others).
You cannot believe how much better the Creative Audigy sound is compared to any on-board sound. I never use on-board sound for anything on my personal computers.
Sound cards are rendered useless in the majority of gaming situations under Windows Vista. See this article
for more information. Anyways, for this reason, as well as the fact that the Audigy has been superseded by the Xtreme series, and the fact that most onboard sound cards are typically very good these days (especially in NVIDIA SLI motherboards, which use hi-definition onboard audio), I suggest skipping the Audigy sound card suggestion here. The money could definitely be spent better in other areas.
Speaking of money, don't be put off by the recommendation of a $2000+ system. You certainly don't need to spend that much for a decent gaming computer. Perhaps give us your budget and maybe a configuration you were considering, and we can take it from there.