Depends on your hardware specs,FYI I find my Vista x64 very stable and have had no problems at all,really impressed how stable it is,I have thrown lots of software and games at it and have found them to all run fine.
I don't think SP1 for Vista will make a lot of difference to tell you the truth,those that have Vista have all the current updates and I would say 98% of the main problems have been fixed,unlike XP which really needed SP1 and even SP2.
Drivers will probably make the main difference with Vista down the road,ie..better gaming performance,hardware support etc...