The best way to look at this is as a deployment issue.
If it IS production, then you should assume you are going to deploy to a NEW server and build it according to your new specs. Then Deploy all of your applications. The means you first need to have deployment packages and even scripts to make life easy. Its a good process to go through anyway to be sure future maintenance will work the way you expect.
If, However, this is a Dev environment then you can look at this as an opportunity to do the right thing since it appears as if you're outgrowing the same-server environment. I would suggest SQL Server and BizTalk should be on separate machines, but you can also split them up on something as small as a laptop using Virtual PC, Virtual Server, or everyone's favorite VM Ware (Workstation or Server).
I'm running an "enterprise" with a Domain Controller, SQL Server, BizTalk 2 server Cluster, and Oracle and MOSS servers as needed. You can run what you need when you need it and test different configurations and scenarios without the time and effort to re-build a new box…especially when you don't have room, budget or you happen to be on the road.
BizTron.