in order from slowest to fastest: SanDisk UltraII, SanDisk ExtremeI, SanDisk ExtremeIII. All 3 of those are faster than any other CF card out there. As far as how long they'll last, well how much thrashing does Win98 do? It's been years since I've used Win98 so I'm not sure, but there may be some registry tweaks you can do to minimize disk writes like the ones we use for XP.
CF card speed comparison: http://www.robgalbraith.com/bins/mul...?cid=6007-6133