Update:
My desktop seems to finally be stabilized with a 2.1 GHz clock. This nVidia nForce2 motherboard apparently does not fully support DDR 400 RAM, as it will only run it at 200 MHz in single-channel mode (even though I have two 1GB modules in slots 1 and 3). My GeForce MX 440 AGP 8x card is running acceptably with ForceWare v93.71, although I already miss the performance of my GeForce Go 6600 Mobile (may it cool in peace).
On that note, setting highpriority = False in your fretsonfire.ini file does indeed help with lag on certain setups. I used to run with it set to True, and got some nice results on my laptop. Since moving to this older desktop, my original FoFiX settings resulted in some bad and uneven lag.
In case anyone else is interested, here is the fretsonfire.ini I ended up with -- it allows me to get an almost constant 60 FPS in single-player modes with my less-than-ideal setup (I had to do this first, and spent a lot of time testing settings - I couldn't code anything with the game not really playable):
http://paste2.org/p/115907(note: scrolling lyrics and sections are disabled -- they are a major source of lag for this older video card. Soon there will be a new MIDI lyric display mode that won't be so demanding.)
So now, finally on my last day off, it looks like I might actually be able to work on FoFiX a little before I have to wake up at 4am for the next 4 days