Get the quad. You know you want it.
Flash games are not multi-threaded so they only run on 1 core. The problem is the rest of your system is also running on the 1 core you have so the quad will allow XP sp3 (assuming you have XP) all the power it needs plus extra cores for Anti-virus, Anti-malware, other apps, and etc....
Flash video is multi-threaded btw.