If your board supports it, Q9550. If not, pickup a Q6600 and OC it to 3ghz on stock cooler. your current Cpu can OC to 3gig as far as I remember though.
I reccomend a quad core for games nowadays over a dual, as most come with support for 4 threads nowadays. just avoid the Q8xxx series like the plague, and if you go for a Q9xxx series, go for the ones with 12mb Cache, its huge.
After seeing your board supports it, go for the Q9550 - if you cant afford one of these, it would have to be an E8400 and an aftermarket cooler. A 4ghz dual core would be plenty enough.
Like said above, you can pick up a q6600 on ebay or a used q955. Sell you current chip and overclock the Q to around 3.0 to 3.2, that will solve your bottle neck issue.
Option 2: depending on your budget would be to sell everything except your power supply, video card, and case. Then buy a motherboard,cpu,and ram upgrade combo. I would recommend doing that if you can, more bang for the buck