So firstly things we can't change easily. Cache the amount of level one, (L1) cache, (the cache in the processor); and the way it's used does make a difference, but there's nothing we can do about that. The more level two,(L2) cache you have, (the cache on the motherboard or on die); does make a difference so if your buying a new motherboard everything else being equal having 1024K L2 cache as opposed to 512K will make a difference. Chances are though, you have your processor and motherboard and aren't about to change them just to get quicker SETI results!
So on to things we can change, BIOS settings! If you have no idea what the BIOS is or does, best to click back now!
Still here... Good :)
As you would expect increasing memory bandwidth, does speed up SETI, remember however the worst crashes you can experience are caused by memory faults, and if you push memory beyond it's limits, unpleasantness can ensue so Backup anything you value before playing <g>.
These tweaks are based on Award BIOS but most will have similar settings so starting in:

Though it's a default and it should be obvious, I'll say it anyway to save any confusion both CPU Internal Cache and External Cache should be ENABLED if they weren't your system would be very slow.
More interesting is BIOS shadowing, it has been suggested that turning this off will help SETI performance, as I don't see any advantage to having it turned on, most graphics drivers will make very few BIOS calls. So Video BIOS Shadow change to DISABLED and any of the other Shadowed BIOS memory ranges, C8000 - CBFFF Shadow etc.
To recap that's:

On to:

Now for the interesting and dangerous stuff! Memory timings in the settings for DRAM timings read as follows: SDRAM 10ns (Default), SDRAM 8ns, Normal, Medium, Fast, Turbo. It doesn't take a genius to work out changing the settings to TURBO gets you a speed increase. My system has been solid with this setting, I have a Samsung PC100 DIMM If you have cheap unbranded memory it may be more of a risk see pc100.com for more on memory.
SDRAM Cycle Length should be set to 2 for PC100 memory, though if you set it to 3 and restart, the BIOS should tell you if your memory should be running at 2.
DRAM Read Pipeline Enabling it increased my memory though put.
Cache R/CPU W pipeline well Enabling it didn't make my system any slower. :)
Cache Timing change it from FAST to FASTEST this does make a noticeable performance difference.
It has been said that caching BIOS slows SETI as it uses cache SETI would use, so Video BIOS Cacheable and System BIOS cacheable set to DISABLED.Finally though it is disabled by default in many BIOSs Spread Spectrum I have been told, is enabled by default in some AmiBIOSs it should be set to DISABLED
To recap that's:

I hope some of this has been of use, and your system is now running quicker, however if whilst playing you have found some settings in your BIOS which have affected a *measurable* improvement on your system, let me know.mail.

Final Warning: This page is intended for people who would play with their BIOS settings anyway, there is some risk in this, and I take no responsibility for losses resulting etc.