Check out sys.dm_io_virtual_file_stats (use parameters NULL,NULL) to get an idea of your current I/O performance since the last restart of SQL Server. If you can identify any bottlenecks here, double-check by checking wait statistics (scripts available on SSC i.e. from Glenn Berry) or use the DMVs for I/O related waits, and use perfmon too (Current Disk Queue Length is a good indicator together with a few others).

Best practice is to split MDF and LDF into separate physical locations but sometimes this is not always possible, e.g. when using LUNs mapped to a SAN or when there aren't enough drives available. Diagnosing poor I/O will give you an indication of where the stress is and which databases need the files moving.

Ideally you'd split tempdb out onto a different volume too.

Check Brent Ozar's training videos page (http://www.brentozar.com/sql-server-training-videos/), specifically 'How to prove it's a SAN problem' and 'Performance 101' for some good advice about I/O-related performance problems.