Your basis in the property will be $420,000 (purchase price) + $100,000(improvement expenses) = $520,000
If you sell the property for $600,000 - you will have $80,000 short term capital gain (held less than a year) which generally will be taxable at your regular tax rate.
Because you owned and use the property as a primary residence for less than two years - you are not eligible to exclude the capital gain from your taxable income
See for reference IRS publication 523 - http://www.irs.gov/pub/irs-pdf/p523.pdf
If you fail to meet the requirements to qualify for the $250,000 or $500,000 exclusion, you may still qualify for a reduced exclusion. This applies to those who:
Fail to meet the ownership and use tests, or
Have used the exclusion within 2 years of selling their current home.
In both cases, to qualify for a reduced exclusion, the sale of your main home must be due to one of the following reasons.
Let me know if you need any help.