Assuming the processes or combination of processes don't ask for more than 256MB, you should easily notice the difference.
Most users regularly go over the 256MB mark (THGC members not included...disclaimer etc), and as memory is inexpensive these days, I'd go for the DDR RAM and then bump it up to 512MB as soon as you can afford to.
If on the other hand, you regularly use a lot more than 256MB (perhaps graphical stuff etc) it might be wise to wait until you have more to swap in.
<b><font color=blue>~ BIOS SETTINGS: Fast, Hot, Unstable...That ought to work. ~</font color=blue></b>