A lot of people have wondered about memory upgrades because Thecus has been pretty quiet about it. The only statement is "... additional memory won't increase system performance ..." or some such nonsense. Yes it will.
My 4100PRO came with 256MB and it SUCKED! I replaced the SO-DIMM with a 1GB module and now I like it. Which SO-DIMM? The Crucial CT12864X40B was what I used but I think any DDR 400 with a CL of 3 will do.
I hope that puts the memory issue to rest. FWIW, stable since 03/11 on FW v3.04.03.