yes and no. you should be happy with 512, it will be enough to run most apps and windows xp... however might be a little slower esp. a couple months from now...
i say no, b/c i can't prommice that putting a second chip of another brand will work without any issues... but it is done, and done everyday. as long as you match the ram as close as possible and update bios, there shouldn't be a problem.
if that's the rout you are thinking, go for it! that's a nice laptop, you should be happy with it!
Edit: just a thought, if you ordered the second module from Dell later on individually by calling them and telling them what you have they will match it up with the ram you need and you won't have a problem.