Tell us what Motherboard you have now, and someone will tell you the fastest processor you can use.
It's not 'as simple as unplug one and plug in another' as the new one may not work in your board. think, when the board was made they could only make it to use processors avaliable at the time they made it. They could not make it to use proessors made the next year, as the processors had not been made yet, so they could not know how to make the board use them (cos they had not been made yet).
the only bios update that can be found on gigabytes website is for rev 5.1 version, heres link click here and it can take athlon2000+ and its over 2 years old, so your bios is the latest and sounds as if you have rev 5.1 version of the ga-7zx but its best to open your case and look on the mobo for its name and under neath should be what version it is.
if you wish to update the bios without checking and you have dual bios on the mobo, you can flash it and if anything goes wrong the secondary bios will boot the pc.