It depends. You might get better advice if you listed your mainboard type.
If you're running a full dual channel system (AMD Socket 939 or an Intel P4
system that isn't low-end), the memory must be added in pairs to preserve
the dual channel operation. Some systems may not operate reliably in single
channel mode with more than a single stick of RAM. (At least, the manual for
my old A8V Deluxe mainboard said that it would not.)
Unfortunately, some mainboards reduce the memory clock if all 4 DIMM sockets
are filled. (AMD systems sometimes drop back to DDR333, from DDR400.) You
may be able to manually set the timing back to DDR400 (PC3200), but the
system may or may not be unstable.
I'm sure an expert would find a great deal of oversimplification in the
above, but I hope that it's all correct, as far as it goes.
My recommendation would be to remove the existing RAM and install two 1 GB
Sticks. That's supposed to be the best point in terms of performance. It's
nowhere near as expensive as it was a year or so ago.
Address scrambled. Replace nkbob with bobkn.