I've done several BIOS mod on my old 7600GT. Flash it about a dozen of time in a week, I want to get the speed right, balancing shader/ROP speed with geometric speeds, but at the end of the day, performance difference is marginal

Editing memory timing is even more complicated, unless you know what you're doing. Not all module can run at certain timings, and with several module brand, things going to go complicated. Judging by my experience, memory overclocking/timing/voltage changes is the easy way to kill a graphics card

Remember anything that can go wrong will go wrong. Prepare a spare PCI card if anything goes wrong. You don't need a floppy, bootable thumbdrive will do nicely, and its faster too

Do lots of research, and really KNOW what you doing before even thinking of flashing. Sometimes its better to just leave it as it is, if it ain't broke don't fix it