Why is windows needed? Just use aticonfig.
There's no need to flash anything for most cards out there.
Many card manufacturers have different hard limits set in the BIOS of the card. This can even vary between individual cards from the same manufacturer.
Out of 5 GigaByte 5870s, I've found 3 different clock limit settings on them:
Memory: 300-1200, 300-1400, 600-1200
Core: 300-900, 300-950, 300-1200
HIS and MSI Lightning's also have their own limits (although my 2 MSI Lightning's allowed obscene overclocking to the point the thing would melt if it didn't crash first).
Flashing the BIOS will remove these imposed limits. aticonfig will not let you go around them.