Bear in mind too that you can recharacterize a conversion (if done by the deadline). As such, if you convert this year, you can have your competent tax professional two draft tax returns: one with the conversion and the other without. If the one 'with' results in more taxes than you can afford, then you can recharacterize all or a part of the conversion amount.
Of course, conversions done in 2010 comes with a wrinkle- as you have the option to include the conversion amount in your 2010 income, or spread the amount 50/50 over 2011 and 2012.
And...what masteff said