The chances are, you will probably be able to have upto a 1Ghz slot A in there, however you will need to do the following.
> Find out what motherboard you have. Crack open the case and look on the motherboard itself. If you don't want get your hands dirty, give Compaq a call and ask them... they can probably tell you how high the current setup will go. Failing that, look on the motherbaord manufaturers website.
> You will need (almost certainly) to update the BIOS of the motherboard to the latest version. As long as its not an unbranded one you should be fine.
With latest BIOS you should be laughing with the whole spectrum of slot a processors (if you can find one