as far as compatibility - you can use PC2100 modules as well as 3200. XP 1800+ runs at 133 (266) Mhz FSB, so if you're planing on running it at stock speeds get yourself PC2700 modules (i don't think PC2100 are still being sold...). But it would be a good idea to buy PC3200 modules if you plan to upgrade your CPU to a 333 or 400 Mhz FSB unit...
Buy PC3200, not much extra money, and it'll allow much room for further upgrades. Plus, 2100 is out of the question -- hard to find unless you're looking at really shitty cheapass ram.
As far as compatibility your motherboard can take any speed of DDR from PC2100 to PC4400.
I believe the multiplier is 11.5 on the 1800+'s??
I would get the PC3200 because it will leave you lots of overclocking headroom to really push the CPU and you wont need to upgrade the RAM if you buy a faster processor and mobo in a few months.