I don't know if it is possible to replace the mobo. Compaq were notorious about changing the hardware. I couldn't tell what form factor it is but I think it's ATX by the dimensions (19.0" x 20.9" x 8.38") It will cost about $200 for cpu, mobo and memory. You will need to upgrade your hard drive if you haven't so already, because Win XP takes up some space. And with the factory drive in (I'm guessing 20Gb) you'll see your space go PDQ.
As for your machine now, you will need to have at least 256Mb RAM. Your machine can only go up to 384Mb (3x 128Mb). I actually checked Compaqs site and that board in your machine only supports up to K7 700Mhz. There are cpus available that can run faster than the maximum speed (evergreen technologies I think).
So really it's up to you. Personally I could make a new one for $500 total including delivery with the components you need. If you are power user (not on it that much, only internet, email and word processing etc) then it will be lower.
HTH
As for your machine now, you will need to have at least 256Mb RAM. Your machine can only go up to 384Mb (3x 128Mb). I actually checked Compaqs site and that board in your machine only supports up to K7 700Mhz. There are cpus available that can run faster than the maximum speed (evergreen technologies I think).
So really it's up to you. Personally I could make a new one for $500 total including delivery with the components you need. If you are power user (not on it that much, only internet, email and word processing etc) then it will be lower.
HTH
But really it's not worth going to the maximum speeds on the original machine or trying to upgrade the mobo and cpu like we said, Compaq made it clear about trying to change the hardware on their machines 
