By Imperialist I'm assuming we're leaving out Canada. So let's start there.
1783 - Bermuda and Bahamas become US territory at the end of the ARW
1819 - purchase Cuba and Florida from Spain
1820s - take Marianas Islands, then Polynesia as a territory
1840s - Not quite 54°40', but the US manages to negotiate 52° N, and creates one new state out of that part of Oregon country
1848 - take Sonora, Chihuahua, Rio Grande, and Baja California in the end of Mexican-American War.
1870s - possibly purchase part or all of Rupert's Land from the UK since the Pacific is pretty much closed off to Canada now, and it becomes more Atlantic / Euro-centric in its foreign policies
1898 - US takes Guam, Philippines in Spanish-American War
1910s - US buys Danish Virgin Islands and British Virgin Islands are given in exchange for US aid in WW1 to the UK.
1940s - Philippines become independent
US has a few new states (58 total) in this scenario:
-Rio Grande, South California, Cuba, Sonora, Sierra Madre (Chihuahua), Virgin Islands, Bahamas, Bermuda
New Territories: Polynesia