BIOS tweaking 
Turbo Frequency - This setting boosts your CPU speed by 2-5% by mildly overclocking your processor. Be careful with this setting, particularly if you are already overclocking, because it could cause some problems with your add-in cards.
QuickPOST - Enable the quick power on self-test to speed up your boot time. The only time you shouldn't have this enabled is if you suspect you are having a hardware problem that it may detect.
CPU L1 and L2 Cache settings - These features should be enabled by default (unless you are using an original 266 or 300 MHz Celeron without an L2 Cache, in which case the L1 cache will be the only one enabled). If someone disabled them on you, make sure you enable them for a substantial performance increase. 

Virus Warning - I personally find this incredibly annoying, but if you have a lot of problems with viruses (hmmm... what have you been downloading?), enabling this can save you a days worth of work recovering your system. You do have a backup, right??? This also can cause some compatibility problems with hardware.
Boot Up Floppy Seek - This setting is pretty antiquated and should be disabled on most new systems (new being Pentium and beyond). It will shave a couple seconds off of the boot up time and will not effect the floppy drives at all. 

PCI/VGA Palate Snoop - This is another antiquated setting that should be disabled unless you have a very old PCI 2D accelerator card. 

Video BIOS Shadow - Turn this off. Most video cards have no use for this feature and it will probably only cause problems. Performance shouldn't be affected much. 

Shadow C8000 - CBFFF, etc - For the most part, you are going to want to leave these settings disabled. If you do know the exact memory ranges for your add-in cards, however, you may be able to boost performance by applying the appropriate settings. 

DRAM Data Integrity Mode - Error Correcting Code mode should be disabled on your system unless you specifically bought that type of RAM. The setting, when used with the right kind of RAM, can increase system stability dramatically. 

Shadow System BIOS - Enabling this can possibly give a system boost, so we recommend that you try it out. Most likely it will not increase performance significantly. 

Video BIOS Cacheable - This setting should increase system performance as well, if your video card is compatible with the setting. However, most newer video cards will not make use of this feature and enabling it can cause problems. 

Video RAM Cacheable - Depending on the system, and how fast the RAM on your video card is, and how much RAM your video card has, this may or may not increase performance. This is a good setting to try with older cards, but if your card has a lot of RAM on it, this setting may decrease performance. 

8 & 16 bit I/O Recovery Time - This setting has to deal with how the computer handles the old ISA bus cards. Setting this to 0 or N/A will usually give the system a slight performance boost. Let it be noted that these setting may cause problems with some ISA cards. 

AGP Aperture Size - This setting truly depends on the video card you have in your system. Check out our Voodoo3, TNT/TNT2, and Geforce tweak guides for specific information on your video cards. 

SDRAM Precharge Control - This should be enabled unless it causes problems with your system. 

Other SDRAM Settings - SDRAM settings, such as CAS and RAS, can definitely increase performance if set properly. However, the lower you set them (lower is better in this case), the more likely your RAM is to fail. The default for these settings is usually 3, but if you feel brave, try out the 2 settings. If they make your system unstable, set them back. Remember, setting your RAM to a CAS or RAS rating they are not spec'd for is akin to overclocking them.
Memory Hole - This is an archaic setting and should be disabled. 

Passive Release - Disabled 

Delayed Transaction - Disabled 

PNP OS - This should be enabled unless you are running Linux on your system as well as a Microsoft OS. If you are having problems with Plug and Play, disabling this may fix the problems. 

Init Display First - Choose PCI or AGP respectively. 

KBC Input Clock - This setting has to do with the Super I/O chip and should be set to 16 MHz.