If you have an i845 SDRAM board and replace it with a nearly identicle i845 DDR SDRAM board you probably will have few if any problems with software. Of course if you were running 9x you could change almost anything without reinstalling the OS simply by removing the existing devices from controll panel.
Yes, dual channel would be a better solution (Granit Bay, Sis 655), but probably not worth the wait considering your CPU.
And there is another problem, you could be using the old Socket423 P4 which is not compatable with newer boards.
<font color=blue>You're posting in a forum with class. It may be third class, but it's still class!</font color=blue>