If you are a start I would go for a dual core server with at least 2 GB of memory. You can always ask your provider if it's easy to expand towards the future.
No reason to spend more money on a big server if you don't use much of the resources in the beginning.
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If you plan to grow quickly, then make sure your solution is scalable or that the company makes the upgrade process as convenient as possible and minimizes any downtime associated with the move. A Core2Quad server processor should meet your needs to start.