If you love the old classics, you can play several great ones on a Pentium 100mHz using Sparcade.  A 400mHz can play many games in DOS mode that it may struggle with in Windows.  Also, try using an older version of MAME such as ver. 036.  I can play Outrun on a 300mHz in DOS mode, so I know your 400mHz can do more than you think it can.
Darryl