512-MByte SDRAM DIMMs are available and cheap too, so those two slots take you up to 1 GByte, which normally should do.
Sure, once the SDRAM-DDR transition has been fully done, fewer and fewer boards will have both types of slots, making room for more DIMM slots.
Anyone remember the transition from PS/2 SIMMs to DIMMs? Same situation, early boards had four SIMM and one DIMM slot, then later only two SIMM slots and two DIMM, until finally the SIMM slots vanished altogether and made room for three or four DIMM slots.
So, if you're updating gradually, move your existing SDRAM over, and if you're building from scratch, then buy ONE large DDR DIMM and still have room for another one.
regards, Peter