I have 2x512 mb sticks installed, but XP Pro only recognizes 768 mb total.
This probably isn't an XP issue. What does the BIOS tell you that you have
installed? Does that state 1024? Download and run AIDA32 from
www.aida32.hu
(don't worry, the site's in English) and see what that tells you. If that
states 768, then there might be a problem with the new stick (or the
motherboard slot - check the manual and see what the largest size is you can
install per slot). It may be that the slot you installed it in can only take
a 256MB stick and won't recognise anything larger. It could be that the
types aren't quite identical and they don't mix well. Are they the same
speed and brand?
If AIDA reports correctly then it's just an XP quirk - sometimes it doesn't
report the RAM correctly, but it will utilise it, if it's there.