|Did you try the CPU yet without making any changes?|
Reliable BIOS? There's only one place - Packard Bell. You could take a chance & try a Gigabyte BIOS but if it doesn't work, you may end up bricking the board in which case it will have to be replaced.
You currently have a 533MHz FSB CPU. According to the following manual, your motherboard does support 800MHz FSB.
You're already running the wrong RAM for your current CPU so if you upgrade to an 800MHz FSB CPU, you should also upgrade to DDR400 RAM (ideally it should be DDR533). The reason being is that the architecture used in the P4 is inefficient. To help makeup for the inefficiency, the CPU:DRAM frequency ratio should be either 3:4, 4:5, or 5:6 rather than the "normal" ratio of 1:1 which is considered optimal for most other CPUs. In other words:
400MHz FSB should use DDR266 (3:4 ratio)
533MHz FSB should use DDR333 (4:5 ratio)
667MHz FSB should use DDR400 (5:6 ratio)
800MHz FSB should use DDR533 (3:4 ratio)
You have 533MHz FSB with DDR266 which is 1:1. If you switch to an 800MHz FSB CPU & keep your current RAM, that ratio would be 3:2.