Last time I checked, DDR RAM was much less expensive than RD RAM. Rambus or RD RAM must be installed in pairs, as well. It's more geared toward server hardware. You won't notice a difference between 256 MB and 512 MB unless you are using multiple applications simultaneously or using software that utilizes a lot of scratch disk or virtual memory.

The Radeon 9700 is currently the best gaming card in the consumer market. The biggest gain in flight simulator software is going to be seen in the video card. Avoid the GeForce MX series for best performance.