Power Mac indicates you are running a machine from pre-2006 with a PowerPC processor. PowerPCs will always have trouble loading the latest websites, as there is no Silverlight for PowerPC, and you probably need to look at these solutions for Adobe Flash:
Power Mac means you have a 601, 603, 604, G3, G4, or G5 in Apple menu -> About This Mac. Intels are not PPC. The reason I mention that, is you are reading from the Power Mac forum. PPCs can run a max out at 10.5.8 if they have at least an 867 Mhz processor (dual 800 Mhz processors don't count).
1 Ghz is higher. The 601 through 604 processors can't run any version of Mac OS X, whereas the G3 that are beige can run up to Mac OS X 10.2.8, The G3 with Firewire can run up to 10.4.11, and the G4 that are slower can run up to 10.4.11.
Java on 10.5.8 is very limited: https://discussions.apple.com/docs/DOC-6491
If you have an Intel, you at least can upgrade to 10.6.8, which is still limited in terms of Java, but a little better:
If you have at least an Intel Core2Duo, Xeon, Core i3, i5, or i7, you can upgrade to Mac OS X 10.7.3 or later, but only after checking your software compatibility with this tip:
Mac OS X 10.7.3 offers both the latest Java and Flash support to date, and thus will not have trouble browsing any website except the few that require Windows, which can be added through this tip:
Apple tower machines with Intel processors are not called PowerMac, but rather are called Mac Pro.