You never add up the core speeds. Duo 2.8 just means the cores run with a 2.8ghz clock rate, where your Core2 runs at 1.8. I run my E6550, which is 2.33ghz by default, overclocked to 2.98 (my current motherboard won't let me reach more, it seems).
pacster: a Core2 at 2.8ghz should let you play mostly at full speed, make sure you have MTGS and Dual Core modes enabled in cpu options.