I would make the Enterprise vs Standard SQL server decision based on the complexity of your need using this chart:
http://www.microsoft.com/Sqlserver/2005/en/us/compare-features.aspxAs you can see, with a database your size you probably won't gain a lot going to Enterprise.
Now where you may see the bottleneck is how many concurrent users and amount of querying being done to determine the processing power and memory needed by the underlying server. That may drive you to question on Windows OS side:
http://msdn.microsoft.com/en-us/library/aa366778.aspx#physical_memory_limits_windows_server_2003As far as the user versus processor licensing, I think I did the analysis once and the per CAL to processor break even point was somewhere around 25 or 50 if I am not mistaken, but I would look at it from that perspective. Especially after determining if your server will have multiple processors as you have to license each physical. One think there is logical doesn't count, so I for example have a Quad Core box but only a single processor based license.
If the $$ make sense, then the processor is always better as it gives you room to grow without extra cost.