Processor speed is an unlikely bottleneck for database-intensive applications.
(Although do realize that you can't compare today's processors with previous generations of chips on a mHz per mHz basis. Your 866 mHz system is probably a Pentium III? A Pentium IV or Athlon 64 is roughly twice as fast as a Pentium III at the same clock speed.)
But I cringe to think what kind of disk drive is likely attached to an old, 866 mHz leftover system. The performance increases in disk drives over the past few years have been substantial.
The latest crop of SATA drives come close to rivaling the hottest SCSI drives, and at a cost not much higher than PATA drives. Of course, your old motherboard is unlikely to support SATA drives, so you will need either an add-in card or a new motherboard.
What OS are you running? With an 866 mHz processor and low memory, I HOPE it's a Linux version, and not Microsoft Windows. That should be plenty of memory for Linux. It's easy to misinterpret Linux memory-usage reporting. If you ARE using Linux, type "free" in a console window. Look at the SECOND line ("-/+ buffers/cache). Linux sucks-up nearly all of available memory for buffers and cache, and gives it back as needed.