Yes, go for quad core. As a matter of facts i too still get lag when running 2 application that nearly used up or is @ 100% cpu usage... Virus scanning the whole pc and other apps that eat a lot of resources.
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Intel Core2Duo E6300 1.86Ghz L629A244 @ 3.78GHz
Asus P5K-E Wifi/AP Rev.1.02g
Thermalright Ultra Extreme 120+ FM121 (110 cfm fan)
ATI x1900 GT Rev.2 256mb 513/657
G.Skill 2x1gb Hz--540MHz
CORSAIR CMPSU-1000HX 1000W