I've been having good luck recently with ATI Tray Tools and the X1900XTX. At first, it wasn't easy figuring out. Hope I'm not stating the obvious:
I've only used the "Hardware" option (the right click "radioactive symbol"). Before you bump up the core, mem - set fan profile to somethign more aggressive (middle tab) then go the to the voltages tab:
ON EACH TAB, MAKE SURE TO CHECK "Include in Profile"
Set GPU Core to 1.425v (Stock 3D mode voltages)
Leave Mem at 2.086 (Stock 3D mode voltages)
Set GPU to 650 and Mem to 775, open the 3D view
I used the artifact scanner and let ir run for a long while, no probs. If you don't have any probs, Save the profile as default. Each time you start Tray Tools, it'll load that profile.
I did find max mem and core - at 1.45/2.088 I'm at 710/810 (not bad). DONT DO THIS UNATTENDED. In some cases, the system crashed (black screen) and once had to recover vis safe mode in XP.
Half-Life looks like a dream at those speeds 1920x1200 everything maxed out!