Will upgrading my 2007 mac-mini from Tiger to Snow Leopard be a performance upgrade?
It is advisable to upgrade to Snow Leopard in your case. More specifically, it's very advisable to upgrade from Tiger to Leopard (10.5) at the very least. This will ensure that you can use newer software and hardware that is on the market these days.

Personally, I didn't notice massive performance increases between Leopard and Snow Leopard, but on paper it is designed to better take advantage of the hardware in your Mac Mini. You'll also see alot of new features that have been incorporated into 10.5 and 10.6 since Tiger.

You'll have to source a copy from eBay or something as now Apple only sell OS X Lion (10.7) through the App Store.