Goofy (unlisted) part numbers usually represent a version that goes to a VAR (value added reselller) or OEM. Finding info is a pain because the board may be brain dead (i.e. some funcitons left off). Or it could be a model meant for another continent (USA, VS European, etc.) Try looking on asus europe, asus TW etc.
If worse comes to worst you can get the numbers for the various chipsets (north and south bridges, LAN, Audio) find an asus board witht he same chipsets and try those drivers. Doing this with the bios is extremely risky.
There are multiple hits on google. I'm glancing through them now.
Edit:
The -LA seems to be a special build for HP systems. One guy says it is similar to -VM MB but with firewire. The VM has:
-400/333/266 FSB
-only goes to PC2700 RAM (strange).
-Thoroughbred and Barton CPUs. No mention of Sempron.
If Semprons can work they will need a new bios. The newest/fastest throroughbred and bartons might need a bios flash also.
Sounds like you are screwed on the upgrade. HP intends that model computer as a throw away with no bios upgrade path.
Best bet would be buy a replacement MB. Or find a MB with a similar bios and try flashing that. But in either case be prepared to buy a new MB. Welcome to the world of HP/Compaq (and all the other "boxed" PCs).