I would suggest you put all your money in a Quad-Core.
Singlecores are badly outnumbered by actual titles.... with resolution for a 19`` monitor your graphiccard still can go 20-30fps with AA off.
Poeple often underestiminate the difference of load to the graphiccard when it comes to wide resolutions on 22´´ and 24´´ Monitors.
By the way...even my 3 GHZ Quad core comes to 100% on all cores in FS X at Frankfurt Airport with 300 AI Planes all over the place...
Andreas