PC2-4300 and PC2-4200 are the same thing it just depends on what manufacturer it is from some round up the speed and label it that way.
From Wikipedia
DDR2 SDRAM gives a transfer rate of (memory clock rate) × 2 (for bus clock multiplier) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte).
This means your PC2-4300/4200 are 133MHz(Memory Clock) x 2 = 266MHz(Bus Clock) x 2 = 533MHz(Data Rate) x 64 = 34112(bits transferred) / 8 = 4264
As you see the 4264 is where PC2-4200/4300 come from an why it depends on who is labeling as some round up and some don't. You can see where the other numbers that are confusing you come into the equation as well.
On older PCs the FSB and ram speeds are tied so yes 533Mhz ram works with your computer and faster ram would work as well it would just be clocked down to the right speed.
On newer PCs you can have different memory ratios so you can run the ram faster than the FSB.