Can I make contributions to a Roth IRA, when I am already contributing to a Roth 401(k) plan? If so, what are the limits?

Thank you,

Yes, you can contribute to both. The limits are are not affected by whether you have a Roth versus traditional IRA or 401(k). For 2009 those limits are:

IRA: $5K, or $6K if age 50 or above. Eligibility for Roth contributions phase out if your AGI is between $166K and $176K (assuming your tax status is married filing jointly). If you exceed these limits then your IRA contribution must be to a traditional IRA.

401(k): $16.5K, or $22K if age 50 or over. This assumes that your company doesn't run afoul of the rules concerning excessive contributions by highly compensated employees.