Montgomery County's Strathmore Hall Arts Center will benefit from a "Roaring Rennaissance Gala" Friday night. For donations of $100 each, patrons will be dinner guests at 20 homes throughout the county, then wind up the evening with a tour of the house and a champagne ball in a large tent on the grounds. Proceeds from the festivities will go to a private endowment for future activiites at the center. Strathmore Hall, the turn-of-the-century Georgian mansion the county purchased as a cultural center in 1978, is to undergo a $685,000 renovation at county expense. This year the manse, on Rockville Pike in North Bethesda, is the National Symphony Decorators' Showhouse. Local interior designers and retailers have decorated 39 rooms, and all are open for tours daily through Nov. 1, for the benefit of the symphony. Visitors may see the rooms weekdays from 10 a.m. until 3 p.m., Saturdays 10 until 4, and Sundays, noon until 5. The house also is open Wednesday evenings from 6 until 8. After the design exhibits, new floor and wall treatments by the decorators will remain as part of the center's permanent decor. For further information or tickets to the art center gala, telephone Louise Lord, 656-6626.