Since you already have 1GB of RAM, the best thing you can do to make your Mac Mini run faster is to replace the stock hard drive with a 7200rpm drive - it will make a difference in all situations where applications are reading or writing to the disk.
It is also possible to experiment with overclocking the CPU and GPU, but I don't recommend it for the average user due to the risk involved, especially with a non-removable CPU/GPU.
The Mini's biggest asset is its low price, so I wouldn't throw too much money into upgrades.