Change from Sand Filter to Cartridge Filter ?

I've read many of the posts about sizing of the filter and remain confused. Given I have and 18x36 pool with a deep end of 8', I estimate 27,000 gallons. I have a 1 1/2 HP pump. I realize bigger is better, but I do want to be reasonable on cost as well. Will a 325 sqft work? Do I need over 400 sqft? I assume I can not go less than 325sqft.

Please help!

Thanks
Rich