You're better to have a dual channel memory setup. It performs better.
Slots 0 & 2 (blue ones) with a matched pair or memory modules.
Either 2x512mb or 2x1gig, depends on what you use your computer for.
2x512mb is fine for most day to day stuff and most games.
Some games and media editing programs work better with more.
If you are going to get more ram... always make sure you can get dual-channel operation. What you are proposing will not achieve this. The only way dual channel mode can be achieved, is with an even number of ram sticks and the sticks must be the same size. Single channel mode signifcantly degrades performance, so get 2 sets of that 1GB kit and keep (or sell) the original stick you had in there, or get 1 kit and leave the original stick out so you're left with 1gb.
Edit: You could also go the route Datman suggested. Either:
1. Get 1 kit and go 2 x 512MB for 1GB.
2. Get 2 kits and go 4 x 512MB for 2GB.
3. Get 1 kit with 2 x 1GB for 2GB total.
One way or another, the orginal 512MB stick you had in there is out of the picture.