Really the MSDE limitation comes down to:
- simultaneous connections to the database at once
- 2Gb size limitation on the database (4Gb for WMSDE and SQL Express)
- MSDE can take advantage of only 2Gb of RAM
- MSDE can take advantage of 2 processors
We actually had all 320 our users on MSDE before our conversion to Notes and moved to SQL - no one experienced any troubles with item delivery (or delivery time), but we weren't heavy attachment users, either.
The more features you use, i.e. the more items that the BES has to translate on its end, the more it will impact performance, so SQL is better in cases like this.
I would monitor your CPU and memory performance of your MSDE server to see how it does during a few normal workdays. My gut feeling (since I don't know the stats of your server) is that most likely, if your box is somewhat respectable, it should do just fine.