I don't know what prices are like in the UK, but here in the states a used G5 dual 2 GHz costs a LOT more than even the most expensive current Mac Mini. And I hate to break it to you, but the current mac mini is more powerful than a dual 2 GHz G5. Check out the Geekbench benchmarks from primatelabs.
Power Mac G5 (Late 2005) PowerPC G5 (970MP) 2.0 GHz (2 cores) = 1805
Mac mini (Mid 2007) Intel Core 2 T7200 2.0 GHz (2 cores) = 2580
http://www.primatelabs.ca/blog/2009/01/mac-performance-january-2009/
I have not looked at these stats, to be honest. I just based on me my own experience - playing with a mac mini in the shop, and my watch. Compared it with a friends Power Mac.
I still love the mini and will be in the queue to buy a new one as soon as one is released (or even the current model at a lower price).
The key selling point for me though was the ability to ramp up the RAM to 8 GB (I have put 6 GB so far) and the ability to replace the video card.
the faster hard drive and oodes of RAM has provided me with a better perfromance.