Here's how you figure out where your bottleneck is (CPU or Graphics). If your game runs smoothly at lower res. or with less options, it's your vid card. If it's always choppy, look at memory and cpu. Great video cards will usually make up for mediocre systems (within reason).
The reason that the Intels kick a$$ in memory performance is the QPB-Quad Pumped Bus. I don't know much about it, but it drastically widens the memory bandwidth-you have nothing to be ashamed of. I think that your system benches where it should. I believe without double-checking that your MBoard is an NF2 right? If that's right, you have two solutions for upgrades that would help. As far as evaluating which is better for you, well that's on you.
1. To get the most of your board/cpu combo, you'll need to open up the FSB. Get yourself two identical PC3200 modules (my experience is they don't have to be dual-channel certified, but it can't hurt) and put them in and adjust your CPU settings to ???x200. A multiplier of 9.5 or 10 should work regardless, 11-11.5 is possible if your temps stay ok and your mobo can up the voltage high enough. Sounds like you're already up there for clock speed, so you must have this part pretty well in hand. The key point is that opening up the FSB from around 133 (PC2100) to 200 (PC3200)will help your memory performance and help your machine do more at the same clock speed.
2. This will probably help more in your flight sim, but it'll cost you more too. Get ahold of a more high-end video card. It's either that or turn off some of the graphics options. Check Tom's 7/2003 VGA Buyer's guide to see the difference between your card and the big dogs-you'll see that it's substantial.
Where there's a will, there's a way-even when it comes to screwing up.