I don't know about gaming or what you will need for those games but if they are fairly recent, £500 will not get a machine with a top class processor. Having said that, ensure that it has an AGP 8x which will be upgradable.
512mb of ram will run photoshop fine.
PowerPC is a type of Mac Processor - generally 1.2ghz PowerPC processors will run at an equivalent of 2.4gh pentium / or 1.8Athlon if I recall correctly.
Remember that Macs use a different OS and are not really gaming machines as are windows based computers.