Upsizing to SQL Server - need advice on version

[Mary]

First of all, I want to thank all of you who post out here regularly. I
have learned so much over the last couple of years! During my 10 years
as a Cobol programmer, working with dozens of other programmers, when I
got stuck, I just had to walk over to the next cubicle to get help. Now
it's just me, which can get overwhelming and quite stressful at
times. So when I need help, I come to you guys. I don't usually even
have to post a message, I just search the group and there's my
answer! Some of you spend a great deal of time walking others through
solutions, and I appreciate that.

Here's my situation: I am employed by a budget-conscious, non-profit
alternative school for children with severe behavior problems. The
school districts pay us to educate their problem students. We currently
have about 300 students enrolled in 5 locations. I have developed a
student database in Access 2003. This database is used to keep track of
student contacts, attendance, therapy hours, unusual incidents that
occur, transportation, classrooms, billing the school districts,
sending out report cards, etc. The database FE and BE, about 45 MB
each, reside on a Windows 2003 server running Terminal Services at the
corporate office. We use Remote Desktop to access the database remotely
from the 5 school buildings. The largest table (with only 3 fields) has
about 55,000 records by the end of the school year and then I archive
it. Other than that, the tables have only up to about 5,000 records. We
generally have about 7 concurrent users, but during report card time we
have had up to 20 without problems. Everything is running great - no
corruptions in 3 years, no problems with record locking or performance.
I am in the position now of having to increase the number of
concurrent users to 30 -40, (and can foresee that number growing) and
am afraid that it's time to bit the bullet and upsize to SQL Server.
The system may be able to handle this many users, but another
consideration is that my boss would like the school districts to be
able to access their students' information from our website, so I'm
thinking upsizing the data would also better position us to do this. I
would like to keep the Access FE mdb and just link to the SQL data.

I know next to nothing about SQL Server, so here is where I need you. I
am trying to determine which version of SQL Server would best serve our
needs. I thought that the free version - SQL Server 2005 Express -
would be great since my company is on a tight budget. The 1 CPU and 4GB
database size limits seem fine. I downloaded it and began playing
around with it, without much success. I went to use DTS to import my
access tables, and quickly realized that DTS is not part of the Express
Edition. So I'm wondering what else it's missing that I might need.
I don't want to make my job more difficult to save a few bucks for my
company. I'm beginning to think the lite version isn't what I want,
and would appreciate any comments.

Thanks in advance for any suggestions.

Mary