Before anything else, make sure your 401(k) will accept rollovers from IRA's.
If you rollover the maximum amount possible from your IRA to a 401(k), this would leave only basis in the Trad IRA.
See this recent thread:
http://benefitslink.com/boards/index.php?s...c=39412&hl=Specifically see page 24 of IRS Pub 590:
http://www.irs.gov/pub/irs-pdf/p590.pdf (underline added for emphasis)
QUOTE
Tax treatment of a rollover from a traditional IRA to an eligible retirement plan other than an IRA. Ordinarily, when you have basis in your IRAs, any distribution is considered to include both nontaxable and taxable amounts. Without a special rule, the nontaxable portion of such a distribution could not be rolled over. However, a special rule treats a distribution you roll over into an eligible retirement plan as including only otherwise taxable amounts if the amount you either leave in your IRAs or do not roll over is at least equal to your basis. The effect of this special rule is to make the amount in your traditional IRAs that you can roll over to an eligible retirement plan as large as possible.
You can then read elsewhere in that publication about doing a conversion to a Roth IRA.