Basically, a 'bottleneck' is the slowest component of a system. The neck of a bottle is generally the narrowest part, so that limits how fast it is to pour anything in to (or out of) it. With computers the situation can change depending on what application you are using, since some programs place more emphasis on the video card, and some are more CPU or memory intensive.
In this case, I don't know which model CPU you have, but the 3 Core 2 Quad CPUs listed in the chart are all good enough to play the game on high details. Similarly, anything over 3GB is listed as good enough for high details. Originally I assumed, based on the model number, your video card was closer to the 3850/4670/3860 ones that were listed in the chart than the 2600 Pro/XT cards, but a quick web search indicates the 4350 may be worse for gaming than the 2600. This means, according to GameStar.de and neglecting optimizations made to D2:ED later, they would expect the game to run smoothly at 1024x786, with minimum Details, no Shadows and no Post-Processing.
A new hard drive may speed up load times, and more memory or a faster CPU may help a bit with games, but with your system a new video card would probably give you the greatest boost in performance. That makes your video card a bottleneck (at least for gaming).
