Is it a bad idea to put Exchange and Sql Server on a same box?
We have about 4-5 GB database and 10 users for exchange, can this work?
Obviously it depends on your load and your hardware, but 10 e-mail users typically doesn't put a lot of load on an exchange server.
So my answer would be yes, it should work just fine.
It should work fine. You won't get much hit on performance with that (of course it depends on ram / cpu you have) assuming you've got decent hardware.
The only drawback of this (just like with any similar SBS 2003/2008 solution) is that when the server goes down both Exchange and SQL go down. But if you can live with some sort of possible downtime then you should be just fine.
To give you an example:
I've got SBS 2003 with MOSS 2007, MS SQL 2005 and soon SQL 2008 R2 Express and Exchange 2003 with 60 users. Hardware is some Xeon with 4GB ram. I do hit problems with big ram usage but mostly because SBS 2003 can't use more then 4GB of ram so I can't even extend the box without buying new license. All in all it works, it works like that for 2 years and I didn't had much problems with it.