Hey,
Firstly, once you go over 512MB, your overall system speed in normal apps actually becomes slightly slower, as your system has to access all this memory that it's not using.
Secondly, it would be silly not to go for a DDR-based solution now. Both the motherboards and the RAM have come down in leaps and bounds. Actually, it was probably the major mistake I made on my system a few months ago (not going DDR) so I hate people making the same mistake.
Hope this helps,
Chris