Time to upgrade, heh???

Well since it seems that i'm the only one who dares to give an opinion I'll try...

Well those memory modules has their own speed that usually is interpreted in MHZ. Normally you would want all your memory modules to have the same speed.
But then came computers that were capable of handling different memory speeds. They would simply force the faster memory modules to work at a slower speed to match the slower memory modules.

So if I was in your shoes I'd simply try with both and see if your computer adds the amount together towards 1500Mb (1,5Gb).
If you see him sticking at 1000Mb(1Gb) or 512Mb, then you should remove the 512 module.

Why didn't you just put 2 or 3Gb of memory?
Windows will wiggle it's tail like a happy dog when it gets much RAM to swim in. Just dont go more than 3Gb.

I hope my answer helped out a little.