before you chose a version you should ask youself some questions like:
how many cals do you need, sbs 2003 allow for i think around 70.
how big will the server load be, will you be having multiple telecomuters accessing resources, will your server be a host file server etc..
what is the server actually going to be used for, this could mean a higher spec machine (for example the hardware requirements for a standard w2k3 install hosting AD is a lot less then a w2k3 box running exchange server).
what hardware you will need for the server to run, this is determined by the above questions answer.
im not sure how big your business is but if you could survive with standard before, then i think that there is no point in moving to enterprise, id check out that link in the previous post, it has all the H/W specs.