Hardware sizing is as much art as science but you need to ask some serious questions about the expected performance of the server before picking a particular server - you mentioned a TB size warehouse, does that include 3-5 years of data growth (or are you going to be keeping a fixed amount of data in the warehouse?), 20-30 concurrent users or 20-30 sometime users? Tools that are going to be used to access the data? Backups? Analysis Services? User expectations as to query performance? Loading/ETL vs. query? It may be you need to have a loading/updatable instance and a reporting instance. The list is long.

As for the differences between a DL380 and a DL580 (or 585), if scalability over time is an issue I'd take a serious look at buying a 580/5 instead of a DL380 - you don't need to load it up (e.g. put only 2 procs in it for now) but it's a lot easier to pop a couple of additional procs/more ram into a machine that already has the extra slots than it is to migrate to a bigger machine (unless you're going to VM).