Dual Raid

Is the Dual Core or Quad Core better than fast single core for games?

My computer seems slow and some games New, and has, Radeon X1950PRO 256MB Graphics Card, Corsair XMS2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 1066 RAM, dual hard drives in RAID 0, Is nForce 680i SLI, and Intel Celeron D 356 Cedar Mill 3.33GHz LGA 775 Processor, push all Vista games like Halo 2 are the definition Laggy Lowest but half life 2 Episode Two run like a charm. What should I replace to fix this?

I have a Centrino Duo and it's okay .. My brother has and he thinks it's core One better.

Dav + Shino, Dual Raid Pwnage