The Mini will be faster than your G5 in every regard. In some respects it may not be massively faster (such as copying files), but in other regards (web browsing, playing videos) it will be much faster.
I strongly recommend upgrading the computer to 4GB of RAM immediately. (If you want to buy from apple, you can do a build-to-order with the RAM already installed for virtually the same price as buying the RAM yourself. At least this was true the last time I checked.)
The imac is definitely faster and if your budget can afford it, it's worth considering. You can correct the reflection problem with anti-glare screen applique. Such as the one from
http://www.RadTech.us