Don't mix the RAM --
I am pretty sure the following is still relivant. If you mix a lower speed ram with a higher speed ram.. your faster ram will clock down to match the crap RAM. Do you really need another 256 MB... NO. I would buy at least a single stick of PC2-3200 DDR2-400 -- should be about $120 and that will be perfect for your system.(that is of course if your motherboard can take DDR2 ~ otherwise just get PC3200 DDR which should be about $50-$60 for 512MB)
I seriously need to do the same myself, I got all this badass shit in my machine and I got like 1GB of friggen lame memory -- my machine has a 400Mhz FSB and I haven't even taken advantage of it yet...
But...
if your FSB is 333 > get PC2700 or better
if your FSB is 366 > get PC3000 or better
if your FSB is 400 > get PC3200 or better
You think they might have just named the memory PC333, PC366 and PC400 huh
More memory is not always good though... Think of it this way, as a person.. being able remeber a crap load of stuff is really cool... but if it takes you forever to recall that information what the hell is the use.
Faster memory = faster computer = less stuff that needs to be put into memory ~~~~ I would take a stick of - PC2-5300 DDR2-667 512MB over a 2GB stick of PC2100 any day of the week.....