Tiger has all the drivers for the Radeon 7000 so don't panic, it's not a great card but it is much better then the 128.

Your G4 can take a mix of PC100 and 133 and you should have no problems (I never have) it will slow the 133 down to the motherboard bus speed which is 100 mhz. As for a RAM tester, first port of call is to go into System Profiler in the Utilities folder. Click on memory and if status is OK and you aren't experiencing any problems then life is good.