The 820 board with RDRAM cannot use SDRAM.
820 with RDRAM is marginally faster than BX with SDRAM. The 820 also has the advantage of offering support for 133 MHz FSB.
If you think you are going to need more than 128 MB of RAM and don't want the new features like AGP 4x, then I'd go for BX. If you aren't going to use more RAM, then go for the 820 with RDRAM.
128 MB is sufficient for most things these days, so I'd say go for 820/RDRAM.