I second what cwa107 said - take a run at OnyX. However, with such a new machine, I would be looking at the applications you have loaded, trying to find out what is starting up automatically and try to disable a few of these. Check your user preference panel and see what your userid starts automatically. If you can, disable a few. This will save memory. Also, per cwa107, go light on the widgets.
Finally, if you can, 1 GB is better. Your mini will breath a lot more easily with a bit more room to move around in. As I recall, the mini ALSO uses its memory for graphics (shared memory graphics) so if you only have 512, and the Mini is using 128 for graphics, you REALLY only have 384 to play with. This is just too little. More RAM really is the best answer, and not all that expensive either. Go for it if you can.