You probably didn't heard me criticising GA-8SR533 a month go. I am currently on this board. It's very stable,but the BIOS feature is "pathetic". The BIOS can't adjust DRAM timing/voltage,CPU voltage,System health only display CPU/System fan speed(no temperature). The board isn't for overclockers though FSB/DRAM speed can be adjusted(it's useless if you can't adjust vcore).
8SRX and 8SR533 differs only in the Southbridge(Both SIS645). 8SRX uses 961 while 8SR533 uses 961B(with ATA-133). 8SRX is better if you are a OCer. Board layout are the same(if you look at their SIS products),but why the BIOS differs so much!!!
I suggest you go for 645DX board,because 645DX has a higher chance of being a "hyperthreading" capable chipset than 645(its newer) and maybe able to do DDR400 as well. Gigabyte's 645DX products are pretty good compare to other manufacturer. They are cheaper comapre to Asus PS533(only NT2900 VS NT 3300)and comes with newer southbridge(962/962L instead of 961B).
If you can afford a wait,Gigabyte's SIS648+963 will be out soon!
gee...the longest post I ever written :D