My 4-year old laptop that cost me < $1000 when I bought it runs SQL 2008 (and 2012) just fine. If you're not planning on doing heavy database work (or are OK with it taking time), just about any reasonable modern machine (other than netbooks) can run SQL Server
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability