The OP could drive over to the Chappaqua station on the Harlem Line, and then take the Harlem Line down to White Plains. The trip will take about 20 minutes (plus driving time)
See this map:
MNR Connecting Service
The Harlem Line schedule is here:
http://www.mta.info/mnr/html/plannin...203%202011.pdf (weekday)
http://www.mta.info/mnr/html/plannin...203%202011.pdf (weekend)
Either that, or the OP can take the Hudson Line to Tarrytown (a 12 minute trip), and then take the BL-13 or TZX (Tappan Zee Express) using a UniTicket.
The schedule is here:
http://www.mta.info/mnr/html/plannin...203%202011.pdf (weekday)
http://www.mta.info/mnr/html/plannin...203%202011.pdf (weekend)
http://transportation.westchestergov.com/images/stories/Schedules/rte_13_timetable.pdf (broken link) (BL-13 schedule)
Here is a Westchester Bus Map: http://transportation.westchestergov.com/images/stories/pdfs/beelinesystemmap.pdf (broken link)
It may not be faster, but it will probably be cheaper (considering the rising gas prices), and more comfortable (you don't have to focus on the road)