This is a good case for using existing older, cheaper memory than expensive ones to save money but even boosting performance.
I doubt you can run PC2100 up to 200 MHz directly in SYNC mode. But you can use the following set up to gain lot of performance.
You can run two sticks of PC2100 (assume you have two sticks) in dual channel ASYNC mode, and you will be getting memory performance like PC2400, or PC2700, ... Actually you are winning on getting more memory bandwidth than just running them in SYNC mode, which most ppl would suggest for AMD MB.
It's been reported that nforce2 MB can run between 180 - 220 MHz FSB out of the box, so it really depends on your "luck".
Since you mentioned A7N8X, I assume your MB can run dual channel. Since I don't know your MB max FSB and CPU, I can just use an example. E.g. assuming your FSB can go to 200 MHz. Then you have to set the CPU multiplier, ...
1. If you plan to run the PC2100 at official speed (133 MHz). Set your ASYNC dual channel at 66% (= 133/200), which is the ratio of the stock freq of the memory to max FSB. I did some analysis, your PC2100 memory would run like a PC2700 (167 MHz) !!! It is only about 18% away in memory BW and 3D performance than using expensive PC3200 memory (2 sticks $200) at 200 MHz SYNC mode.
2. If you find out you can overclock your PC2100 to PC2400 (which is doable if the memory is good). Then you shall set your ASYNC dual channel as 75% (= 150/200), which is the ratio of the memory frequency to max FSB. I did some analysis, now your memory would run like a PC2800 (175 MHz) !!! It is only about 15% away in memory BW and 3D performance than using expensive PC3200 memory (2 sticks $200) at 200 MHz SYNC mode.
Save $140 for other parts !!!
In case you can overclock your PC2100 even higher, you can do similar calculation to get closer to the target PC3200 (200 MHz) performance. E.g. ASYNC 80%, but sometimes this setting may not be very stable, have to test out.
When you get your MB, first find out its max FSB and do the calculation and bios setting accordingly.
Details about some analysis of the nforce2 dual channel ASYNC mode performance can be found in
this link