Just a bit of a warning vhoong but I had a similar problem with my 256 Mb SD card (i also have a 4150) and I think it is related to certain brands of card. I had a 128 card made by kodak and never had problems. I recently got the 256 (at a good price) and have been having a heap of problems.
I formatted card at least 3 times to get rid of the corruptions but they would come back if I tried to copy and paste files from PC straight onto the storage card while my ppc was synced. I don't think this should have been a problem as I did it with my old card and had no problems. The stuff I was copying was mp3's and ebooks. Just as long as I did not do file operations then I would not get any corruptions.
If you do a check on google (I cannot remember my search terms), then there are quite a few people who have had a problem. I even came across a site which posted good/problem cards.
http://sdprob.aximsite.com/theproblemlist.htm
I have since gone back to the 128 and have had no problems (and made sure no file operatiosn from pc to ppc).
Hope this helps,
JBB