The memory should work fine, I'd drop it to be safe (unless you can get confirmation that it works) and get a 2GB (2 x 1GB) Crucial Ballistix DDR2 800 deal for $125, I prefer Corsair and Crucial for ram. And no you don't have any bottlenecks, I don't think it will last any more than 3 years of gaming without an upgrade, but that's really besides the point.
More than $190 for any dual core is a waste. The Q6600 for $300 is about an 80% increase over a E6600. That said the best dual core the X6850 would only be a 20% increase in performance over the E6600. The Q6600 is a much better bang for the buck than E6750 for only $75 more.
In non quad compatible games you may see a small advantage for the E6750 as your graphic drivers will use the quad core anyway. The best part is the Q6600 can OC about the same as E6750 for the average user. Most will see about 3.3GHz from both.