Yes, CAPITALISE is a Scrabble word!

Letter values in Scrabble US: C 3, A 1, P 3, I 1, T 1, A 1, L 1, I 1, S 1, E 1.

Is it a valid word?

CAPITALISE is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

supply with capital, as of a business by using a combination of capital used by investors and debt capital provided by lenders
draw advantages from
write in capital letters
compute the present value of a business or an income
consider expenditures as capital assets rather than expenses
convert (a company's reserve funds) into capital

Also: take advantage.